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/gaming.nix

@@ -51,7 +51,7 @@ in
       # };
     };
     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 = builtins.attrValues {
         inherit retroarchWithCores;
         inherit (pkgs)
@@ -70,8 +70,6 @@ in
           prismlauncher # minecraft launcher with jdk overlays
           ;
       };
-    in lib.mkMerge (map (user: {
-      ${user}.packages = packages;
-    }) users);
+    in userLib.mkUserPackages config.my.toggleUsers.apps packages;
   };
 }