Add helper functions for multi-user toggle support

Introduced a new file `lib.nix` containing helper functions to streamline user package management and attributes for multi-user configurations. Updated various modules to utilize these functions, enhancing code maintainability and readability.

modules/apps/art.nix

@@ -40,9 +40,7 @@ in
     dev.gameDev.enable = lib.mkEnableOption "game development tools and engines";
   };
   config.users.users = let
-    users = if lib.isString config.my.toggleUsers.apps then [ config.my.toggleUsers.apps ] else config.my.toggleUsers.apps;
+    userLib = import ../lib.nix { inherit lib; };
     packages = artPackages ++ gameDevPackages;
-  in lib.mkMerge (map (user: {
-    ${user}.packages = packages;
-  }) users);
+  in userLib.mkUserPackages config.my.toggleUsers.apps packages;
 }