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/dictionaries.nix

@@ -5,7 +5,7 @@
   ...
 }:
 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 (pkgs)
       hunspell
@@ -20,8 +20,6 @@ in
 {
   options.my.apps.dictionaries.enable = lib.mkEnableOption "dictionaries and language tools";
   config = lib.mkIf config.my.apps.dictionaries.enable {
-    users.users = lib.mkMerge (map (user: {
-      ${user}.packages = packages;
-    }) users);
+    users.users = userLib.mkUserPackages config.my.toggleUsers.apps packages;
   };
 }