lib/options: add TODO regarding lua-types

`defaultNullOpts` is generally used for representing lua-options.

We should probably a) rename it something like `luaOptions` and switch
out the primitive types to use the new `lua-types`.

lib/options.nix

@@ -114,6 +114,7 @@ rec {
     { type, ... }@args: mkNullOrOption' (args // { type = with types; either strLuaFn type; });
   mkNullOrStrLuaFnOr = type: description: mkNullOrStrLuaFnOr' { inherit type description; };
 
+  # TODO: use lib.nixvim.lua-types
   defaultNullOpts =
     let
       # Ensures that default is null and defaultText is not set