HTML API: Add new is_self_contained() method to Tag Processor.

[dmsnell]

This new method will help calling code trying to determine whether or not to expect to find a closing tag, since the Tag Processor consumes the entire element (including the closing tag) when it encounters one.

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[sirreal]

This function seems helpful, part of the reason I believe that is that is seems to roughly to some of the special conditions here in the HTML Processor:

wordpress-develop/src/wp-includes/html-api/class-wp-html-processor.php

Lines 795 to 806 in 06f7c82

	return ! (
	// Comments, text nodes, and other atomic tokens.
	'#' === $token_name[0] ||
	// Doctype declarations.
	'html' === $token_name ||
	// Void elements.
	self::is_void( $token_name ) ||
	// Special atomic elements.
	( 'html' === $token_namespace && in_array( $token_name, array( 'IFRAME', 'NOEMBED', 'NOFRAMES', 'SCRIPT', 'STYLE', 'TEXTAREA', 'TITLE', 'XMP' ), true ) ) ||
	// Self-closing elements in foreign content.
	( 'html' !== $token_namespace && $token_has_self_closing )
	);

Do you think we can and should adapt this method to support our use case there?

[dmsnell]

@sirreal this is seeming more complicated than I want it to be. specifically the interaction with $token in the HTML Processor. I'll table it again until we can ensure there's a good clean interaction with expects_closer(), which itself needs review.