wrappers: make helpers available via lib option

NixOS, HM, & Darwin provide a `lib` option, not to be confused with the
`lib` module arg.

This option is intended for modules to define custom functions without
having to extend nixpkgs lib.

The old option is renamed, if it is accessed it will print:

    trace: Obsolete option `nixvim.helpers' is used. It was renamed to `lib.nixvim'.

docs/user-guide/helpers.md

@@ -10,12 +10,12 @@ If Nixvim is built using the standalone method, you can access our `helpers` as
 ```
 
 If Nixvim is being used as as a home-manager module, a nixos module, or as a dawwin module,
-helpers can be accessed via the `config.nixvim.helpers` option:
+helpers can be accessed via the `config.lib` option:
 
 ```nix
 { config, ... }:
 let
-  inherit (config.nixvim) helpers;
+  helpers = config.lib.nixvim;
 in
 {
   # Your config