feat(gnome): add enable option

modules/desktops/gnome/default.nix

@@ -1,9 +1,13 @@
+{ lib, config, ... }:
+
 {
+  options.desktop.gnome.enable = lib.mkEnableOption "GNOME desktop";
+
+  config = lib.mkIf config.desktop.gnome.enable {
     services.xserver = {
       displayManager.gdm.enable = true;
       displayManager.gdm.wayland = true;
       desktopManager.gnome.enable = true;
     };
-
-  # TODO: Declare monitors.xml
+  };
 }