Add contains method to open elements

From WordPress#6972

src/wp-includes/html-api/class-wp-html-open-elements.php

@@ -101,6 +101,24 @@ public function set_push_handler( Closure $handler ) {
 		$this->push_handler = $handler;
 	}
 
+	/**
+	 * Reports if a node of a given name is in the stack of open elements.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @param string $node_name Name of node for which to check.
+	 * @return bool Whether a node of the given name is in the stack of open elements.
+	 */
+	public function contains( string $node_name ): bool {
+		foreach ( $this->walk_up() as $item ) {
+			if ( $node_name === $item->node_name ) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+
 	/**
 	 * Reports if a specific node is in the stack of open elements.
 	 *