HTML API: Backport updates from Core (#57022)

Updates from WordPress/wordpress-develop at 54a09a7ec99c59b8e640bd5aacebfdbf03bb02cc

 - WordPress/wordpress-develop#5535
   Adds support for H1 - H6 elements in the HTML Processor.

 - WordPress/wordpress-develop#5725
   Pause the Tag Processor when reaching the end of the document
   inside an incomplete syntax element.

 - Incorporates linting changes from "TODO:" to "todo"

This patch adds the blanket exclusion from the HTML API compatability
layer after they were removed and started blocking updates from Core.

The PHP files in the compatability layer are merged and maintained in
the Core repo and all changes or updates need to happen first in Core
and then be brought over to Gutenberg as built files. From Gutenberg's
perspective they are no different than NPM packages.

Co-authored-by: Anton Vlasenko <43744263+anton-vlasenko@users.noreply.github.com>
Co-authored-by: Bernie Reiter <96308+ockham@users.noreply.github.com>