Ensure layout classnames are applied to the inner blocks wrapper

[tellthemachines]

Trac ticket: https://core.trac.wordpress.org/ticket/65101

Syncs WordPress/gutenberg#77408.

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Props isabel_brison, andrewserong.

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[andrewserong]

Very tiny nit, but should we add the trac ticket number to the test_layout_support_flag_renders_classnames_on_wrapper test, since the data provider now has a new entry?

[andrewserong]

This looks good to me! It correctly syncs the Gutenberg PR (WordPress/gutenberg#77408) and is a good iterative improvement over trunk (skip void elements and tags with a closer).

While in the linked PR there was a suggestion to investigate using WP_HTML_Processor (instead of WP_HTML_Tag_Processor), the explorations into doing so proved worse in terms of performance than this approach, so this seems like the right one to get in to me.

Also, if there's a desire to revisit the approach, there's still plenty of time to come back to it in the 7.1 cycle. So LGTM to land 🚀

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 62504
GitHub commit: c8f7e7d

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.