Add has_bookmark method

lib/compat/wordpress-6.3/html-api/class-gutenberg-html-tag-processor-6-3.php

@@ -1481,6 +1481,18 @@ private function apply_attributes_updates() {
 		$this->lexical_updates = array();
 	}
 
+	/**
+	 * Whether a bookmark with the given name exists.
+	 *
+	 * @since 6.3.0
+	 *
+	 * @param string $bookmark_name Name to identify a bookmark that potentially exists.
+	 * @return bool Whether that bookmark exists.
+	 */
+	public function has_bookmark( $bookmark_name ) {
+		return array_key_exists( $bookmark_name, $this->bookmarks );
+	}
+
 	/**
 	 * Move the internal cursor in the Tag Processor to a given bookmark's location.
 	 *