lib: provide an "extended lib" to our modules

lib/extend-lib.nix returns a nixpkg's lib extended with our own
helpers.

This is exposed as `helpers.extendedLib`, but when evaluating our
modules it should be assigned to `specialArgs.lib`.

Outside of our modules you must still access our helpers via
`config.lib.nixvim` or `config.lib.nixvim.extendedLib`.

Within helpers' sub-sections, `lib` is the extended lib.

wrappers/standalone.nix

@@ -31,6 +31,7 @@ let
         ];
         specialArgs = {
           inherit helpers;
+          lib = helpers.extendedLib;
           defaultPkgs = pkgs;
         } // extraSpecialArgs;
       };