Generalize anonymous box creation to table box kinds

[nicoburns]

Summary

Step 0 of the anonymous table box fixup work (split out from #783; based on #781's branch): generalizes anonymous node creation without changing behavior.

LayoutChildren::create_anonymous_block's body is extracted into a free function parameterized by kind:

pub(crate) enum AnonKind { Block, Table, TableRow, TableCell }

pub(crate) fn create_anonymous_node(doc, container_node_id, kind: AnonKind) -> NodeId
// kind selects the precomputed pseudo: ServoAnonymousBox / ServoAnonymousTable /
// ServoAnonymousTableRow / ServoAnonymousTableCell

Since precomputed pseudos take their declarations from UA stylesheet rules, default.css gains

*|*::-servo-anonymous-table { display: table; }
*|*::-servo-anonymous-table-row { display: table-row; }
*|*::-servo-anonymous-table-cell { display: table-cell; }

so the table-kind anonymous boxes (used by #783 on top of this) carry honest computed displays. The Block path is a pure refactor; the table variants are unused here (#[allow(dead_code)] until #783 lands on top).

cargo fmt / clippy clean; existing table/anonymous-cell tests pass.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/e0b69caedcd34e829ff73fb7ff216a75
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/e0b69caedcd34e829ff73fb7ff216a75?variant=devin-insiders
Requested by: @nicoburns

WPT results

71 newly passing, 19 newly failing (net +52).

Full diff (90 changed tests)

+ Fail => Pass css/CSS2/backgrounds/background-applies-to-015.xht
+ Fail => Pass css/CSS2/backgrounds/background-attachment-applies-to-015.xht
+ Fail => Pass css/CSS2/backgrounds/background-color-applies-to-015.xht
+ Fail => Pass css/CSS2/backgrounds/background-image-applies-to-015.xht
+ Fail => Pass css/CSS2/backgrounds/background-repeat-applies-to-015.xht
- Pass => Fail css/CSS2/bidi-text/bidi-008a.xht
- Pass => Fail css/CSS2/bidi-text/bidi-008b.xht
+ Fail => Pass css/CSS2/borders/border-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-bottom-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-bottom-color-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-bottom-width-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-color-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-left-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-left-color-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-left-width-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-right-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-right-color-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-right-width-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-top-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-top-color-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-top-width-applies-to-015.xht
+ Fail => Pass css/CSS2/borders/border-width-applies-to-015.xht
+ Fail => Pass css/CSS2/colors/color-applies-to-015.xht
- Pass => Fail css/CSS2/css21-errata/s-11-1-1b-001.html
- Pass => Fail css/CSS2/css21-errata/s-11-1-1b-008.html
- Pass => Fail css/CSS2/css21-errata/s-11-1-1b-009.html
+ Fail => Pass css/CSS2/fonts/font-applies-to-015.xht
+ Fail => Pass css/CSS2/fonts/font-variant-applies-to-015.xht
+ Fail => Pass css/CSS2/fonts/font-weight-applies-to-015.xht
+ Fail => Pass css/CSS2/linebox/vertical-align-applies-to-015.xht
+ Fail => Pass css/CSS2/lists/list-style-applies-to-015.xht
+ Fail => Pass css/CSS2/lists/list-style-image-applies-to-015.xht
+ Fail => Pass css/CSS2/lists/list-style-type-applies-to-015.xht
+ Fail => Pass css/CSS2/margin-padding-clear/margin-applies-to-015.xht
+ Fail => Pass css/CSS2/margin-padding-clear/margin-left-applies-to-015.xht
+ Fail => Pass css/CSS2/margin-padding-clear/margin-right-applies-to-015.xht
+ Fail => Pass css/CSS2/margin-padding-clear/margin-top-applies-to-015.xht
+ Fail => Pass css/CSS2/margin-padding-clear/padding-applies-to-015.xht
+ Fail => Pass css/CSS2/margin-padding-clear/padding-bottom-applies-to-015.xht
+ Fail => Pass css/CSS2/margin-padding-clear/padding-left-applies-to-015.xht
+ Fail => Pass css/CSS2/margin-padding-clear/padding-right-applies-to-015.xht
+ Fail => Pass css/CSS2/margin-padding-clear/padding-top-applies-to-015.xht
+ Fail => Pass css/CSS2/normal-flow/height-applies-to-015.xht
- Pass => Fail css/CSS2/normal-flow/inline-table-002a.xht
- Pass => Fail css/CSS2/normal-flow/inline-table-width-002b.xht
+ Fail => Pass css/CSS2/normal-flow/inline-table-zorder-002.xht
+ Fail => Pass css/CSS2/normal-flow/max-height-applies-to-015.xht
+ Fail => Pass css/CSS2/normal-flow/max-width-applies-to-015.xht
+ Fail => Pass css/CSS2/normal-flow/min-height-applies-to-015.xht
+ Fail => Pass css/CSS2/normal-flow/min-width-applies-to-015.xht
+ Fail => Pass css/CSS2/normal-flow/width-applies-to-015.xht
+ Fail => Pass css/CSS2/tables/anonymous-table-box-width-001.xht
+ Fail => Pass css/CSS2/text/letter-spacing-applies-to-015.xht
+ Fail => Pass css/CSS2/text/text-decoration-applies-to-015.xht
+ Fail => Pass css/CSS2/text/text-indent-applies-to-015.xht
+ Fail => Pass css/CSS2/text/white-space-applies-to-015.xht
+ Fail => Pass css/CSS2/text/word-spacing-applies-to-015.xht
- Pass => Fail css/CSS2/ui/outline-applies-to-016.xht
- Pass => Fail css/CSS2/ui/outline-applies-to-017.xht
- Pass => Fail css/css-break/table/break-before-second-row.html
- Pass => Fail css/css-break/table/caption-margin-002.html
- Pass => Fail css/css-break/table/caption-margin-005.html
+ Fail => Pass css/css-break/table/inside-flex-001.html
+ Fail => Pass css/css-break/table/monolithic-overflow-002.tentative.html
+ Fail => Pass css/css-break/table/monolithic-overflow-004.tentative.html
+ Fail => Pass css/css-break/table/repeated-section/image.tentative.html
+ Fail => Pass css/css-contain/contain-inline-size-table.html
- Pass => Fail css/css-contain/contain-size-056.html
- Pass => Fail css/css-contain/contain-size-table-caption-001.html
- Pass => Fail css/css-contain/content-visibility/content-visibility-094.html
- Pass => Fail css/css-contain/content-visibility/content-visibility-095.html
+ Fail => Pass css/css-flexbox/flexbox_flex-formatting-interop.html
+ Fail => Pass css/css-flexbox/flexbox_stf-table-singleline-2.html
+ Fail => Pass css/css-flexbox/flexbox_stf-table-singleline.html
- Pass => Fail css/css-flexbox/table-as-item-flex-cross-size.html
+ Fail => Pass css/css-flexbox/table-as-item-narrow-content.html
- Pass => Fail css/css-flexbox/table-as-item-stretch-cross-size-2.html
- Pass => Fail css/css-flexbox/table-as-item-stretch-cross-size.html
+ Fail => Pass css/css-flexbox/table-with-float-paint.html
+ Fail => Pass css/css-grid/grid-items/explicitly-sized-grid-item-as-table.html
+ Fail => Pass css/css-page/monolithic-overflow-009-print.html
+ Fail => Pass css/css-page/monolithic-overflow-010-print.html
+ Fail => Pass css/css-page/monolithic-overflow-011-print.html
+ Fail => Pass css/css-page/monolithic-overflow-017-print.html
+ Fail => Pass css/css-sizing/calc-margins-table-caption.html
+ Fail => Pass css/css-sizing/table-child-percentage-height-with-border-box.html
+ Fail => Pass css/css-sizing/table-percentage-max-width-beside-float.html
+ Fail => Pass css/css-tables/html-display-table.html
+ Fail => Pass css/css-tables/percent-height-overflow-auto-in-restricted-block-size-cell.html
+ Fail => Pass css/css-tables/percent-height-overflow-auto-in-unrestricted-block-size-cell.tentative.html

_{Generated by the WPT workflow.}

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