Add baseline post format support

[justintadlock]

Currently, there is not much support for post formats in block themes, as outlined in this ticket:

• Bring post formats to block themes WordPress/gutenberg#53049

However, there are some foundational things we could explore to add some basic theme support:

• Add a custom filter to the Query Loop block to query by post format.
• "Register" existing post format archive templates with a custom title and description that appear in the UI.
• Filter the template hierarchy to add single post format templates.
• Register the single post format templates in the UI.
• Filter the template hierarchy to remove the post-format- slug prefixes and cleanup the template names (e.g., taxonomy-post-format-image.html would be nicer than taxonomy-post_format-post-format-image.html).
• Add a variation to the Post Terms block for the post_format taxonomy.

[justintadlock]

To get post format support for the Query Loop block, I enabled rest_api for the post format taxonomy (didn't add it to the theme because this is Core/plugin territory):

add_filter('register_post_format_taxonomy_args', function($args) {
	$args['show_in_rest'] = true;
	return $args;
});

This works on the front end. But in the editor, the spinner just keeps spinning and never loads posts (maybe there's a need for a custom REST controller):

I could dive into this more and add custom controls to the Query Loop block, but I'll leave this for the time being. May revisit later.