General: Add block_hooks field to block type registration, REST API.

In order to implement Block Hooks, we need to add a new `block_hooks` field to the `WP_Block_Type` class, as well as to block registration related functions, the block types REST API controller, etc.

Props gziolo.
Fixes #59346. See #59313.

git-svn-id: https://develop.svn.wordpress.org/trunk@56587 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/blocks.php

@@ -342,6 +342,7 @@ function get_block_metadata_i18n_schema() {
  * @since 5.9.0 Added support for `variations` and `viewScript` fields.
  * @since 6.1.0 Added support for `render` field.
  * @since 6.3.0 Added `selectors` field.
+ * @since 6.4.0 Added support for `blockHooks` field.
  *
  * @param string $file_or_folder Path to the JSON file with metadata definition for
  *                               the block or path to the folder where the `block.json` file is located.
@@ -513,6 +514,39 @@ function register_block_type_from_metadata( $file_or_folder, $args = array() ) {
 		}
 	}
 
+	if ( ! empty( $metadata['blockHooks'] ) ) {
+		/**
+		 * Map camelCased position string (from block.json) to snake_cased block type position.
+		 *
+		 * @var array
+		 */
+		$position_mappings = array(
+			'before'     => 'before',
+			'after'      => 'after',
+			'firstChild' => 'first_child',
+			'lastChild'  => 'last_child',
+		);
+
+		$settings['block_hooks'] = array();
+		foreach ( $metadata['blockHooks'] as $anchor_block_name => $position ) {
+			// Avoid infinite recursion (hooking to itself).
+			if ( $metadata['name'] === $anchor_block_name ) {
+				_doing_it_wrong(
+					__METHOD__,
+					__( 'Cannot hook block to itself.', 'gutenberg' ),
+					'6.4.0'
+				);
+				continue;
+			}
+
+			if ( ! isset( $position_mappings[ $position ] ) ) {
+				continue;
+			}
+
+			$settings['block_hooks'][ $anchor_block_name ] = $position_mappings[ $position ];
+		}
+	}
+
 	if ( ! empty( $metadata['render'] ) ) {
 		$template_path = wp_normalize_path(
 			realpath(