add new external flake to provide helper functions

modules/features/nixos/stylix.nix

@@ -6,12 +6,7 @@
 
     options.features.nixos.stylix.enable = lib.mkEnableOption "stylix";
 
-    config = let
-      mkIfElse = with lib; p: yes: no: mkMerge [
-        (mkIf p yes)
-        (mkIf (!p) no)
-      ];
-    in lib.mkIf (config.features.nixos.stylix.enable) {
+    config = lib.mkIf (config.features.nixos.stylix.enable) {
       stylix.enable = true;
       stylix.image = builtins.path { path = "${self}/modules/hosts/${config.info.hostName}/wallpaper.jpg"; };
       stylix.polarity = "dark";