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.

flake-modules/packages.nix

@@ -1,4 +1,8 @@
-{ inputs, ... }:
+{
+  inputs,
+  helpers,
+  ...
+}:
 {
   perSystem =
     {
@@ -9,6 +13,7 @@
     }:
     {
       packages = import ../docs {
+        inherit helpers;
         inherit system;
         inherit (inputs) nixpkgs;
         inherit (inputs') nuschtosSearch;