HTML API: Treat </br> as tag opener in more places

[sirreal]

</br> gets special treatment in the HTML standard. It creates a BR element with no attributes (regardless of whether it appears to have attributes in the HTML like </br attr-name>).

• Stop on </br> in ::next_tag() like other openers.
• Return array() from ::get_attribute_names_with_prefix() on </br> instead of null like true close tags.
• Add tests.

To do:

• Ensure </br> works with functions like ::set_attribute() and ::add_class(). These will require rewriting the tag to <br attrs …> or the attributes will be discarded.

Trac ticket: https://core.trac.wordpress.org/ticket/65372

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[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 61197
GitHub commit: 03171e1

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.

[dmsnell]

@desrosj looks like this is another victim of over-eager bots closing work that wasn’t merged. this patch was related to the Core ticket mentioned in the SVN commit, but the actual changeset was orthogonal to it.

[sirreal]

Moved to trac ticket https://core.trac.wordpress.org/ticket/65372, this has been appearing in fuzz testing. I added a to-do, this likely needs some extra work to handle attribute modification. It will basically need to rewrite the entire token span I think because any attribute-like syntax needs to be removed before rewriting to <br> and only then can attributes be added.

I would like to move this forward when those issues are handled.