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/shell/config.nix

@@ -15,10 +15,10 @@
   };
   config = {
     users.users = let
-      users = if lib.isString config.my.toggleUsers.shell then [ config.my.toggleUsers.shell ] else config.my.toggleUsers.shell;
+      userLib = import ../lib.nix { inherit lib; };
     in lib.mkMerge (map (user: {
       ${user}.shell = pkgs.${config.my.shell.type};
-    }) users);
+    }) (userLib.normalizeUsers config.my.toggleUsers.shell));
     programs.zsh.enable = config.my.shell.type == "zsh";
   };
 }