Blocks: Change traverse_and_serialize_block(s)'s callback signature.

During work on #59399, it was discovered that ''sibling'' block insertion wasn't likely going to work the way it was planned, which required devising an alternative solution. This new solution requires some changes to `traverse_and_serialize_block(s)`:

- Change the signature of the existing callback such that:
  - the return value is a string that will be prepended to the result of the inner block traversal and serialization;
  - the function arguments are: a ''reference'' to the current block (so it can be modified inline, which is important e.g. for `theme` attribute insertion), the parent block, and the previous block (instead of the block index and chunk index).
- Add a second callback argument to `traverse_and_serialize_block(s)`, which is called ''after'' the block is traversed and serialized.
  - Its function arguments are a reference to the current block, the parent block, and the next block.

Props gziolo.
Fixes #59412. See #59313.
Built from https://develop.svn.wordpress.org/trunk@56644


git-svn-id: http://core.svn.wordpress.org/trunk@56156 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/block-template-utils.php

@@ -515,16 +515,15 @@ function _inject_theme_attribute_in_block_template_content( $template_content )
  * @access private
  *
  * @param array $block a parsed block.
- * @return array Updated block.
+ * @return void
  */
-function _inject_theme_attribute_in_template_part_block( $block ) {
+function _inject_theme_attribute_in_template_part_block( &$block ) {
 	if (
 		'core/template-part' === $block['blockName'] &&
 		! isset( $block['attrs']['theme'] )
 	) {
 		$block['attrs']['theme'] = get_stylesheet();
 	}
-	return $block;
 }
 
 /**