Fix parsing of class names like 'w-1/3' as used in selectors like '.w-1\/3'

[westonruter]

Summary

• Update PHP-CSS-Parser to 134f4e6...bc6ec74 on master.
• Remove patches/php-css-parser-138-extended.patch (introduced in Require unreleased version of PHP-CSS-Parser #4300) which was merged into master.
• Update patch to fix parsing CSS selectors which contain commas, originally in Fix parsing CSS selectors which contain commas MyIntervals/PHP-CSS-Parser#138 but now updated at Fix parsing CSS selectors which contain commas westonruter/PHP-CSS-Parser#1.

Fixes #4609

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[westonruter]

Build for testing: amp.zip (v1.5.4-alpha-20200420T213813Z-70cee0e15)

[schlessera]

Note that SabberWorm PHP CSS Parser seems to use PSR-2/PSR-12, not WPCS, so code style needs to be adapted if this is to be merged into the upstream package.

[pierlon]

Otherwise from @schlessera's note above, this looks good 👍 .

[westonruter]

I'll keep this to the v1.6 release since updating the CSS parser could have big impacts.