Upgrade php-css-parser to v8.4.0

[westonruter]

See changelog for v8.4.0.

PHP-CSS-Parser went a long time without a release. Version 8.3.0 was released 2019-02-22, and in order to deploy unreleased improvements we updated to include the commits we needed (#4300, #5081) in addition to applying additional patches (#5392, #4290, etc).

Changes between v8.3.0 and what we released: MyIntervals/PHP-CSS-Parser@8.3.0...bfdd976
Changes between what we released and v8.4.0: MyIntervals/PHP-CSS-Parser@bfdd976...8.4.0

So this PR updates to the latest stable release as well as:

• Remove now-obsolete php-css-parser-install-composer-plugin.
• Bump stylesheet cache group.
• Update patches to address merge conflicts.

Additional patches on top of v8.4.0 are applied from these PRs:

• Replace invalid turns unit with turn MyIntervals/PHP-CSS-Parser#350
• Validate name-start code points for identifier westonruter/PHP-CSS-Parser#2
• Fix parsing CSS selectors which contain commas westonruter/PHP-CSS-Parser#1

[github-actions]

Plugin builds for ee6f7c7 are ready 🛎️!

• Download development build
• Download production build

[dhaval-parekh]

Changes looks good to me.

[westonruter]

Eesh. I just found that the patches fail to apply when running composer install --prefer-dist but they succeed when running composer install --prefer-source.

[westonruter]

Yeah, it's also failing in the GHA build:

  - Applying patches for sabberworm/php-css-parser
    https://github.com/sabberworm/PHP-CSS-Parser/compare/e41d2140031d533348b2192a83f02d8dd8a71d30...b1f74798a7d21d0e6f6402ee8a06c92c7f92fae5.diff (1. Add additional validation for size unit <https://github.com/sabberworm/PHP-CSS-Parser/pull/350>)
   Could not apply patch! Skipping. The error was: Cannot apply patch https://github.com/sabberworm/PHP-CSS-Parser/compare/e41d2140031d533348b2192a83f02d8dd8a71d30...b1f74798a7d21d0e6f6402ee8a06c92c7f92fae5.diff
    https://github.com/westonruter/PHP-CSS-Parser/compare/e41d2140031d533348b2192a83f02d8dd8a71d30...a49bd92c35e9bbbaaca23cf33874a65dd485c30b.diff (2. Validate name-start code points for identifier <https://github.com/westonruter/PHP-CSS-Parser/pull/2>)
   Could not apply patch! Skipping. The error was: Cannot apply patch https://github.com/westonruter/PHP-CSS-Parser/compare/e41d2140031d533348b2192a83f02d8dd8a71d30...a49bd92c35e9bbbaaca23cf33874a65dd485c30b.diff
    https://github.com/westonruter/PHP-CSS-Parser/compare/e41d2140031d533348b2192a83f02d8dd8a71d30...935ea79e0389e672a9deadf9f366e6faab9110ba.diff (3. Fix parsing CSS selectors which contain commas <https://github.com/westonruter/PHP-CSS-Parser/pull/1>)
   Could not apply patch! Skipping. The error was: Cannot apply patch https://github.com/westonruter/PHP-CSS-Parser/compare/e41d2140031d533348b2192a83f02d8dd8a71d30...935ea79e0389e672a9deadf9f366e6faab9110ba.diff

I believe it's due to the patches including changes to tests which aren't included in dist.

[maitreyie-chavan]

@dhaval-parekh Please could you review this again further to the findings @westonruter shared and new changes pushed.