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/dev/julia.nix

@@ -26,9 +26,7 @@ in
   };
   config = lib.mkIf config.my.dev.julia.enable {
     users.users = let
-      users = if lib.isString config.my.toggleUsers.dev then [ config.my.toggleUsers.dev ] else config.my.toggleUsers.dev;
-    in lib.mkMerge (map (user: {
-      ${user} = { inherit packages; };
-    }) users);
+      userLib = import ../lib.nix { inherit lib; };
+    in userLib.mkUserAttrs config.my.toggleUsers.dev { inherit packages; };
   };
 }