feat: add app handler method, get_appdir()

docs/glossary.rst

@@ -25,6 +25,9 @@ Glossary
      Usually this is named ``app`` and is located at the root of the
      virtual environment.
 
+     Can be retrieved via
+     :meth:`~wuttjamaican.app.AppHandler.get_appdir()`.
+
    app enum
       Python module whose namespace contains all the "enum" values
       used by the :term:`app`.  Available on the :term:`app handler`