feat(phpactor): override the default template

Make classes final and enable strict types by default.

Refs: #41

config/phpactor/phpactor.yml

@@ -1,4 +1,5 @@
 code_transform.class_new.variants:
+  default: default
   drupal-functional-test: drupal-functional-test
   drupal-kernel-test: drupal-kernel-test
   drupal-unit-test: drupal-unit-test

config/phpactor/templates/default/SourceCode.php.twig

@@ -0,0 +1,10 @@
+<?php
+
+declare(strict_types=1);
+
+namespace {{ prototype.namespace }};
+
+{% for class in prototype.classes %}
+final class {{ class.name }} {
+}
+{% endfor %}