Add LastModifiedExtensionInterface and implementation in AbstractExtension to track modification of runtime classes

src/Extension/LastModifiedExtensionInterface.php

@@ -0,0 +1,23 @@
+<?php
+
+/*
+ * This file is part of Twig.
+ *
+ * (c) Fabien Potencier
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Twig\Extension;
+
+interface LastModifiedExtensionInterface extends ExtensionInterface
+{
+    /**
+     * Returns the last modification time of the extension for cache invalidation.
+     *
+     * This timestamp should be the last time the source code of the extension class
+     * and all its dependencies were modified (including the Runtime class).
+     */
+    public function getLastModified(): int;
+}