Block support: Add server-side processing for anchor

[wildworks]

Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/74183

We are adding anchor (id) support to dynamic blocks. Dynamic blocks are rendered server-side, so we need to process the ID attribute value to output it.

Trac ticket: https://core.trac.wordpress.org/ticket/64449

[wildworks]

Testing instructions

WordPress core does not yet have any dynamic blocks that support anchors, so run the following code to add anchor support to the Archive block:

add_filter( 'register_block_type_args', function( $args, $block_name ) {
        if ( 'core/archives' === $block_name ) {
                $args['supports']['anchor'] = true;
        }
        return $args;
}, 10, 2 );

• Open a post.
• Switch to Code Editor mode and enter the following HTML: <!-- wp:archives {"anchor":"my-anchor"} /-->
• Save the post.
• On the frontend, the Archive block should have an ID attribute.