HTML API: Add spawn fragment parser method

[sirreal]

Work in progress

Trac ticket: https://core.trac.wordpress.org/ticket/62357

The HTML API has two ways of creating HTML processors, create_fragment and
create_full_parser.

create_fragment is a static method that creates a fragment parser from a very reasonable default context node, a BODY element.

However, it may be necessary to create a fragment parser with a different context node, for example when working with HTML inside of an SVG or TABLE tag may be parsed very differently.

One strong motivation for this is the implementation of a set_inner_html method which requires parsing an HTML fragment with the appropriate context node in order to change its contents.

Add a spawn_fragment_parser( string $html_fragment ) method. This method must be called when the processor is paused at a #tag with some strict constraints:

• Must be paused on a #tag.
• The opening and closing tags must appear in the HTML input (no virtual tokens).
• No "self-contained" elements are allowed ( IFRAME, SCRIPT, TITLE, etc.).

 OK, but incomplete, skipped, or risky tests!
-Tests: 1500, Assertions: 1079, Skipped: 421.
+Tests: 1686, Assertions: 1222, Skipped: 464.

This supports work on set_inner_html in #7326 and #7742.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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