lib: make overrideable & access via flake

Add a non-system specific `<flake>.lib.nixvim` output, which is
equivalent to the existing `<flake>.lib.<system>.helpers` output.

This is now also wrapped with `lib.makeOverridable` to allow overriding
the function args used to construct the nixvim-lib.

Consistently access nixvim-lib via the new flake output, overriding
where necessary.

docs/default.nix

@@ -1,4 +1,5 @@
 {
+  helpers,
   system,
   nixpkgs,
   nuschtosSearch,
@@ -7,7 +8,6 @@ let
   # We overlay a few tweaks into pkgs, for use in the docs
   pkgs = import ./pkgs.nix { inherit system nixpkgs; };
   inherit (pkgs) lib;
-  helpers = import ../lib { inherit lib; };
 
   nixvimPath = toString ./..;