fix: Enable helpers to be passed as an argument in nixvim (#688)

With this change `helpers` can be imported using `{pkgs, lib, helpers,
...}` instead of requiring to `import` it.

flake.nix

@@ -99,7 +99,7 @@
           packages =
             {
               docs = pkgs-unfree.callPackage (import ./docs) {
-                modules = nixvimModules;
+                modules = modules pkgs;
               };
             }
             // (import ./helpers pkgs);