status/darkMode: Add dark mode toggle

The aggregate menu with its submenus isn't well-suited for simple
on-off actions, so we didn't expose the global color-scheme support
that was introduced last cycle.

Quick settings on the other hand are a natural fit for actions like
this, so add a corresponding toggle.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2392>

js/js-resources.gresource.xml

@@ -135,6 +135,7 @@
     <file>ui/status/accessibility.js</file>
     <file>ui/status/autoRotate.js</file>
     <file>ui/status/brightness.js</file>
+    <file>ui/status/darkMode.js</file>
     <file>ui/status/dwellClick.js</file>
     <file>ui/status/location.js</file>
     <file>ui/status/keyboard.js</file>