Editor: Add block theme infrastructure

Adds the required infrastructure to render block-based themes. This is sourced
from the Gutenberg plugin.

Fixes #54335.
Props bernhard-reiter, youknowriad, ntsekouras, hellofromtonya.

Built from https://develop.svn.wordpress.org/trunk@52062


git-svn-id: http://core.svn.wordpress.org/trunk@51654 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/blocks.php

@@ -1154,6 +1154,8 @@ function build_query_vars_from_query_block( $block, $page ) {
  *
  * It's used in QueryPaginationNext and QueryPaginationPrevious blocks.
  *
+ * @since 5.9.0
+ *
  * @param WP_Block $block   Block instance.
  * @param boolean  $is_next Flag for hanlding `next/previous` blocks.
  *