Move Nix files into a nix directory

Move everything from `config` to the root level.

phpactor/templates/drupal-unit-test/SourceCode.php.twig

@@ -0,0 +1,13 @@
+<?php
+
+declare(strict_types=1);
+
+namespace {{ prototype.namespace }};
+
+use Drupal\Tests\UnitTestCase;
+
+{% for class in prototype.classes %}
+final class {{ class.name }} extends UnitTestCase {
+
+}
+{% endfor %}