extract stylix to separate module, add nixos module with auto-import

nix/nixos-module.nix

@@ -0,0 +1,22 @@
+self:
+{
+  config,
+  options,
+  lib,
+  ...
+}:
+let
+  cfg = config.programs.nova-shell;
+in
+{
+  options.programs.nova-shell.enable = lib.mkEnableOption "nova-shell Quickshell bar";
+
+  config = lib.mkIf cfg.enable (
+    lib.optionalAttrs (options ? home-manager) {
+      home-manager.sharedModules = [
+        self.homeModules.default
+      ]
+      ++ lib.optionals (options ? stylix) [ self.homeModules.stylix ];
+    }
+  );
+}