Adding rel="me" to WordPress social link block items

wordpress_social_rel.php

<?php
/**
 * Mastodon accounts can be verified by adding a rel="me" link in the basic rendered html.
 * While a <link/> could be added, it seemed like the existing social-links block should be
 * the place to add it, but there's currently no way to specify "rel" values in the UI.
 *
 * Currently only adds to "mastodon" links, but could probably be added to others.
 */
add_filter('render_block', 'social_rel_me', 10, 2);
function social_rel_me($block_content, $block) {
  if ( $block['blockName'] === 'core/social-link' ) {
    if ( $block['attrs']['service'] === 'mastodon' ) {
      $block_content = preg_replace('/<a /', '<a rel="me" ', $block_content);
    }
  }
  return $block_content;
}