Build/Test Tools: Add a workflow for the HTML API's html5lib tests.

[costdev]

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.

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

[github-actions[bot]]

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Props costdev, jonsurrell, desrosj, jorbin.

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[costdev]

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

[github-actions[bot]]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• The Plugin and Theme Directories cannot be accessed within Playground.
• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance, it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[sirreal]

There's an alternative approach in https://github.com/WordPress/wordpress-develop/pull/7602 leveraging the reusable phpunit tests.

[desrosj]

Let's close out in favor of #7602.