minor #10025 Add missing RuntimeExtensionInterface (tjamps, javiereguiluz)

This PR was submitted for the 4.1 branch but it was merged into the 3.4 branch instead (closes #10025).

Discussion
----------

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.")

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Commits
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4e9b044 Added the missing versionadded directive
cb102c1 Remove duplicate `use` keyword
0e8d863 Add missing RuntimeExtensionInterface

Author: javiereguiluz

templating/twig_extension.rst

@@ -115,7 +115,9 @@ previous ``priceFilter()`` method::
     // src/AppBundle/Twig/AppRuntime.php
     namespace AppBundle\Twig;
 
-    class AppRuntime
+    use Twig\Extension\RuntimeExtensionInterface;
+
+    class AppRuntime implements RuntimeExtensionInterface
     {
         public function __construct()
         {
@@ -132,51 +134,12 @@ previous ``priceFilter()`` method::
         }
     }
 
-Register the Lazy-Loaded Extension as a Service
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-Finally, register your new class as a service and tag it with ``twig.runtime``
-(and optionally inject any service needed by the Twig extension runtime):
-
-.. configuration-block::
-
-    .. code-block:: yaml
-
-        # app/config/services.yml
-        services:
-            app.twig_runtime:
-                class: AppBundle\Twig\AppRuntime
-                public: false
-                tags:
-                    - { name: twig.runtime }
-
-    .. code-block:: xml
-
-        <!-- app/config/services.xml -->
-        <?xml version="1.0" encoding="UTF-8" ?>
-        <container xmlns="http://symfony.com/schema/dic/services"
-            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-            xsi:schemaLocation="http://symfony.com/schema/dic/services
-                http://symfony.com/schema/dic/services/services-1.0.xsd">
-
-            <services>
-                <service id="app.twig_runtime"
-                    class="AppBundle\Twig\AppRuntime"
-                    public="false">
-                    <tag name="twig.runtime" />
-                </service>
-            </services>
-        </container>
-
-    .. code-block:: php
-
-        // app/config/services.php
-        use AppBundle\Twig\AppExtension;
+ .. versionadded:: 3.4
+     The ``RuntimeExtensionInterface`` was introduced in Symfony 3.4.
 
-        $container
-            ->register('app.twig_runtime', AppRuntime::class)
-            ->setPublic(false)
-            ->addTag('twig.runtime');
+If you're using the default ``services.yaml`` configuration, this will already
+work! Otherwise, :ref:`create a service <service-container-creating-service>`
+for this class and :doc:`tag your service </service_container/tags>` with ``twig.runtime``.
 
 .. _`Twig extensions documentation`: http://twig.sensiolabs.org/doc/advanced.html#creating-an-extension
 .. _`global variables`: http://twig.sensiolabs.org/doc/advanced.html#id1