Framework: Support nested templates

[youknowriad]

This PR adds support for nested templates. For example, a CPT could use a columns block in its templates and assign a template for this nested block.

This uses the third argument of the array defining a block in a template as a way to define a sub-template.

This doesn't address nested locking yet. This will be worked on separately.

Testing instructions

• Define a block type with nested templates like this:

function register_book_type() {
	$args = array(
		'public' => true,
		'label'  => 'Books',
		'show_in_rest' => true,
		'template' => array(
			array( 'core/image' ),
			array( 'core/paragraph', array(
				'placeholder' => 'Add a book description',
			) ),
			array( 'core/quote' ),
			array( 'core/columns', array(), array(
				array( 'core/image', array( 'layout' => 'column-1' ) ),
				array( 'core/paragraph', array(
					'placeholder' => 'Add a inner paragraph',
					'layout' => 'column-2' 
				) ),
			) )
		),
	);
	register_post_type( 'book', $args );
}
add_action( 'init', 'register_book_type' );

• Check that when inserting a new book, the template is initialized with a prefilled columns block.

[mtias]

Should subTemplate be innerBlocks?

[aduth]

Nice 👍

Minor point: I'd prefer consistency on "inner blocks" naming than introducing "sub" as a concept, e.g. innerBlocksTemplate.

[mtias]

This is something we should mention on the templates doc.