feat: add COSMIC specialisation

desktop/cosmic.nix

@@ -0,0 +1,17 @@
+{ inputs, lib, ... }:
+
+{
+  imports = [ inputs.nixos-cosmic.nixosModules.default ];
+
+  # TODO at the moment COSMIC cannot be used alongside Gnome
+  # https://github.com/lilyinstarlight/nixos-cosmic/issues/17
+  specialisation = {
+    cosmic.configuration = {
+      services.xserver.displayManager.gdm.enable = lib.mkForce false;
+      services.xserver.desktopManager.gnome.enable = lib.mkForce false;
+
+      services.desktopManager.cosmic.enable = true;
+      services.displayManager.cosmic-greeter.enable = true;
+    };
+  };
+}