Refactor: Avoid use folder for one file

Fix #3

modules/system/desktop/gnome.nix

@@ -0,0 +1,29 @@
+{ config, lib, ... }:
+
+with lib;
+let
+  cfg = config.modules.system.desktop.gnome;
+in
+{
+  options.modules.system.desktop.gnome = {
+    enable = mkEnableOption ''
+      Enable gnome with my custom configurations
+    '';
+
+    enableGdm = mkOption {
+      type = types.bool;
+      description = "Enable gdm with custom gnome";
+      default = true;
+    };
+  };
+  config = mkIf cfg.enable {
+    # Enable the X11 windowing system.
+    services.xserver.enable = true;
+
+    # Enable the Gnome Desktop Environment.
+    services.xserver.displayManager.gdm.enable = cfg.enableGdm;
+
+    # Enable the GNOME shell.
+    services.xserver.desktopManager.gnome.enable = true;
+  };
+}