Add missing RuntimeExtensionInterface

If the Twig runtime class does not implement RuntimeExtensionInterface, the autoconfigure feature of the container will not tag it with `twig.runtime`, and an exception will be thrown ("Unable to load the "App\Twig\AppRuntime" runtime.")

Author: tjamps

templating/twig_extension.rst

@@ -117,7 +117,9 @@ previous ``priceFilter()`` method::
     // src/Twig/AppRuntime.php
     namespace App\Twig;
 
-    class AppRuntime
+    use use Twig\Extension\RuntimeExtensionInterface;
+
+    class AppRuntime implements RuntimeExtensionInterface
     {
         public function __construct()
         {