[2.1] Add Composer plugin to correctly autoload PHP CSS Parser dependency

[pierlon]

Summary

Backport of #6464.

Checklist

• 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).

[github-actions]

Plugin builds for bca6e68 are ready 🛎️!

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• Download production build

[pierlon]

I've traced the E2E JS test failures to the latest Gutenberg release (v11.1.0). Whenever a test navigates to the Onboarding Wizard screen the Gutenberg plugin throws an error on the page because the admin bar is not present on the page, which causes puppeteer to lose its mind and bail for some reason. I've filed an issue on their repo (see WordPress/gutenberg#33591) to see how best that can be resolved.

In the interim I think we can temporarily fix these E2E tests by using and pinning the last Gutenberg release (v11.0.0), as it won't throw the error.

Also note this affects the develop branch as well.

[pierlon]

Also note this affects the develop branch as well.

Weirdly puppeteer doesn't bail and runs all tests to completion, although it does emit similar errors:

That's most likely due to the develop branch using a much more recent version of puppeteer.

[westonruter]

Also note this affects the develop branch as well.

Will you open a PR to do this for develop as well?

[pierlon]

Yep sure.

[westonruter]

QA Passed

In the 2.1 branch, the autoload path is:

amp-wp/composer.lock

Lines 202 to 212 in 4205e6e

	"require-dev": {
	"codacy/coverage": "^1.4",
	"phpunit/phpunit": "^4.8.36"
	},
	"default-branch": true,
	"type": "library",
	"autoload": {
	"psr-4": {
	"Sabberworm\\CSS\\": "lib/Sabberworm/CSS/"
	}
	},

This corresponds with #6464 (comment).