parse rgba/hsla/hsva with optional alpha (#274)

[cpruijsen]

CSS Color 4 (the grammar quoted in #274) makes the alpha group on rgba() optional:

rgba( [ <number> | <percentage> | none ]{3} [ / [ <alpha-value> | none ] ]? )

TinyColor required a fourth CSS unit, so rgba(255, 0, 0) did not parse (isValid() === false, color acted like black). Same matcher is used for hsla() / hsva().

Fixes #274

Summary

• PERMISSIVE_MATCH4 (shared by rgba / hsla / hsva) now takes three components with an optional fourth.
• / is accepted as the separator before that optional alpha, matching the modern form in the issue (rgba(255 0 0 / 0.5)).
• Omitted alpha still becomes 1 through the existing boundAlpha path. Four-value comma forms are unchanged.

Decision

• Chose: optional fourth component on the shared *a matcher, including slash-separated alpha.
• Alternative: rgba-only, and/or leave slash-separated alpha invalid.
• Why: those three parsers already share one regex. Making the fourth optional without / would parse rgba(255 0 0 / 0.5) as opaque red, because the matchers are unanchored and leftover input is ignored. Happy to drop slash, or to limit this to rgba, if you prefer.

Generated UMD/CJS/ESM copies are produced by deno task build and are not in this commit. I can add a build commit if you want the CDN copies updated here.

Test plan

• rgba(255, 0, 0) and rgba 200 100 0 parse (isValid(), equals the RGB object, alpha 1 via toRgbString)
• rgba(90%, 45%, 0%) without alpha still parses
• rgba(255 0 0 / 0.5) keeps alpha 0.5
• rgba(255, 0, 0, 0.5) / rgba 200 100 0 .4 (existing four-value) still work
• hsla(...) / hsva(...) without alpha parse
• deno test test.js, 45 passed, 1 ignored