run html-api-html5lib-tests grouped tests for changes to html api

[jorbin]

Follow up to #60227 and [58010].

The html-api-html5lib-tests group of tests is excluded by default since it contains many intentionally and dynamically skipped tests (see discussion in #60227 for why this is the best route for these tests).

These tests should be run when there is a change to the HTML API related files though so that it's not up to maintainers to remember to manually run them.

This adds a GitHub workflow to run the HTML API's html5lib tests when any changes are made to the HTML API, the html5lib tests, or the html5lib data files. Runs on multiple versions of PHP.

@desrosj Do you have time to take a look at this in case there's anything I missed?

Trac ticket: https://core.trac.wordpress.org/ticket/61209.

[desrosj]

I just realized that I had a Slack DM with @costdev (and I think @jorbin at some point) and never followed up with a summary here. Apologies for that!

I am strongly of the opinion that this should not have a separate workflow, and instead somehow integrated into the regular PHPUnit one. Since the idea is to only run these when the related files are updated, I suggested we add a flag to run this group of tests to a preexisting job and use something like tj-actions/changed-files to detect changes.

I've opened a PR that is an extension of @costdev's initial one and our discussion, but still is not quite detecting changes correctly, but is along what I had in mind.

For now I am punting this to 6.7.