Implement extended filtering for language matching

[KiChjang]

#15464 added support for the lang pseudo-class, but it is using an equality comparison to check the language of the element with the languages specified in :lang(). We should instead use the algorithm specified here for matching languages.

Spec: https://tools.ietf.org/html/rfc4647#section-3.3.2
Files: components/script/dom/element.rs, components/script/layout_wrapper.rs
Tests: tests/wpt/web-platform-tests/dom/nodes/Element-matches.html, tests/wpt/css-tests/selectors-3_dev/, tests/wpt/web-platform-tests/html/dom/elements/global-attributes/ (use ./mach test-wpt with the appropriate file path to run the tests)

[emilio]

Do you think this is an E-Easy or E-less-easy issue @KiChjang?

[KiChjang]

Could be!

[highfive]

Hi! If you have any questions regarding this issue, feel free to make a comment here, or ask it in the #servo channel in IRC.

If you intend to work on this issue, then add @highfive: assign me to your comment, and I'll assign this to you. 😄

[SimonSapin]

It is https://drafts.csswg.org/selectors/#the-lang-pseudo (currently at "Level 4") that specified complex BCP 47 matching.

An easy intermediate step is to implement Level 3 of the spec: https://drafts.csswg.org/selectors-3/#lang-pseudo

Whether an element is represented by a :lang() selector is based solely on the element's language value […] being equal to the identifier C, or beginning with the identifier C immediately followed by "-" (U+002D). The matching of C against the element's language value is performed case-insensitively within the ASCII range.

[sendilkumarn]

I would like to give it a shot. This will be my first rust code

@highfive: assign me

[highfive]

Hey @sendilkumarn! Thanks for your interest in working on this issue. It's now assigned to you!