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/exercism.nix

@@ -8,15 +8,13 @@
   options.my.shell.exercism.enable = lib.mkEnableOption "Exercism coding practice platform";
   config = lib.mkIf config.my.shell.exercism.enable {
     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; };
       packages = builtins.attrValues {
-      inherit (pkgs)
-        exercism # learn to code
-        bats # testing system, required by Exercism
-        ;
+        inherit (pkgs)
+          exercism # learn to code
+          bats # testing system, required by Exercism
+          ;
       };
-    in lib.mkMerge (map (user: {
-      ${user}.packages = packages;
-    }) users);
+    in userLib.mkUserPackages config.my.toggleUsers.shell packages;
   };
 }