SSR: Experiment with WP_HTML_Tag_Processor

[ockham]

Trimmed-down version of #118, based directly on the main-wp-directives-plugin branch.

• Adds PHP unit test infrastructure and CI.
• Adds WP_HTML_Tag_Processor files from Gutenberg trunk (i.e. everything from GB's lib/experimental/html/ is copied to src/html/). Now relies on the GB plugin to be installed and active, and uses its files directly.

We then add individual directives into src/directives/, and their corresponding unit tests into phpunit/directives/.

This should allow us to adopt a test-driven development approach, as we implement all missing directives, and to identify what other features we need in WP_HTML_Tag_Processor (also see #120).

✅ - Implemented
🟡 - Waiting for an upcoming Gutenberg release to include a missing feature
❌ - Blocked by missing feature in WP_HTML_Tag_Processor

Name	Attribute directive	Tag directive	Missing feature
wp-context		✅
wp-show	❌	❌	Wrapping content inside HTML tags with extra tags. E.g. get_content_inside_balanced_tags() from WordPress/gutenberg#46345, and a matching set_content_inside_balanced_tags() (see WordPress/gutenberg#47036).
wp-bind	✅	N/A	get_attribute_names_with_prefix(); landed in GB 15.0
wp-class	✅	N/A	See wp-bind.
wp-style	✅	N/A	See wp-bind. Furthermore, see WordPress/gutenberg#46887. (The latter isn't a blocker, though.)
wp-text			Replace content between HTML tags. See wp-show.
wp-for / wp-each
wp-html			Replace content between HTML tags. See wp-show.
wp-slot & wp-fill

Testing Instructions

Note that this PR currently requires at least GB v14.9. You might have to npm run wp-env destroy and restart it in order to update your GB version.

In addition, this PR currently uses get_attribute_names_with_prefix from WordPress/gutenberg#46840, which hasn't been merged into GB yet.

To make it available locally, either add a .wp-env.override.json and point it to your local GB directory (with the required branch checked out), or apply the patch to your ~/.wp-env/<yourdirectory>/gutenberg/ folder.

[ockham]

I'll close this PR, since most of its code has now been merged into the main-wp-directives-plugin branch (per #133) 🎉
For wp-show (which is still blocked), I've opened #141.

I'll carry the status table from the PR description over to the tracking issue (#120), which will then serve as the only source of truth.