feat(justfile): add phpunit recipe

Add a `phpunit` recipe for running PHPUnit tests, with a `test` alias
that could later be used for running Pest in PHP projects or Jest in a
node project.

templates/justfile.twig

@@ -6,6 +6,11 @@ default:
 {% if "php" is same as language %}
 composer *args:
   {{ "just _exec php composer {{ args }}" | raw }}
+
+alias phpunit := test
+
+test *args:
+  {{ "just _run phpunit {{ args }}" | raw }}
 {% endif %}
 
 {% if "drupal-project" is same as type %}