Block Supports: Strip custom CSS from blocks for users without edit_css capability

[glendaviesnz]

Adds capability-gated CSS stripping so that when a user without edit_css saves a post, any style.css attributes are surgically removed from block comments using WP_Block_Parser::next_token().

Trac ticket: https://core.trac.wordpress.org/ticket/64771

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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.

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[github-actions]

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[ramonjd]

This is working according to the test steps in WordPress/gutenberg#76650 for me (minus the editor warning of course, which is part of the GB packages). Code matches the GB filters and tests.

I'll rebase for final review.

[ramonjd]

You can also test that author's can't post block CSS to the REST API :

wp.apiRequest( {
	path: '/wp/v2/posts/<YOUR_POST_ID>',
	method: 'POST',
	contentType: 'application/json',
	processData: false,
	data: JSON.stringify( {
		content:
			'<!-- wp:paragraph {"style":{"css":"color:green"}} --><p class="has-custom-css">Welcome to WordPress. This is your first post. Edit or delete it, then start writing!</p><!-- /wp:paragraph -->',
	} ),
} ).done( console.log ).fail( console.error );

[ramonjd]

I added these tests to cover that the filters are added, which I think is the core guarantee of this feature?