modules/lua-loader: make enable option nullable

This avoids having the option always "defined".

This also avoids luaLoader configuration in extra files by default.
Earlier `vim.loader.disable()` was always added to configs produced
by `files` option, effectively disabling luaLoader even if it was
explicitly enabled in a top-level configuration.

modules/lua-loader.nix

@@ -1,30 +1,30 @@
-{ lib, config, ... }:
-with lib;
+{
+  lib,
+  config,
+  helpers,
+  ...
+}:
 let
   cfg = config.luaLoader;
 in
 {
-  options.luaLoader = {
-    enable = mkOption {
-      type = types.bool;
-      default = false;
-      description = ''
-        Whether to enable/disable the experimental lua loader:
+  options.luaLoader.enable = helpers.mkNullOrOption lib.types.bool ''
+    Whether to enable/disable the experimental lua loader:
 
-        If `true`: Enables the experimental Lua module loader:
-          - overrides loadfile
-          - adds the lua loader using the byte-compilation cache
-          - adds the libs loader
-          - removes the default Neovim loader
+    If `true`: Enables the experimental Lua module loader:
+      - overrides loadfile
+      - adds the lua loader using the byte-compilation cache
+      - adds the libs loader
+      - removes the default Neovim loader
 
-        If `false`: Disables the experimental Lua module loader:
-          - removes the loaders
-          - adds the default Neovim loader
-      '';
-    };
-  };
+    If `false`: Disables the experimental Lua module loader:
+      - removes the loaders
+      - adds the default Neovim loader
 
-  config = {
+    If `null`: Nothing is configured.
+  '';
+
+  config = helpers.mkIfNonNull' cfg.enable {
     extraConfigLuaPre = if cfg.enable then "vim.loader.enable()" else "vim.loader.disable()";
   };
 }