neovim: Make it possible to configure using Lua

nixos/modules/programs/neovim.nix

···
       example = lib.literalExpression ''
         {
           customRC = '''
-            " here your custom configuration goes!
+            " here your custom VimScript configuration goes!
+          ''';
+          customLuaRC = '''
+            -- here your custom Lua configuration goes!
           ''';
           packages.myVimPackage = with pkgs.vimPlugins; {
             # loaded on launch

pkgs/applications/editors/neovim/utils.nix

···
   makeNeovimConfig =
     {
       customRC ? "",
+      customLuaRC ? "",
       # the function you would have passed to lua.withPackages
       extraLuaPackages ? (_: [ ]),
       ...
···
     attrs
     // {
       neovimRcContent = customRC;
+      luaRcContent = customLuaRC;
       wrapperArgs = lib.optionals (luaEnv != null) [
         "--prefix"
         "LUA_PATH"
···
           vimAlias
           ;
         customRC = configure.customRC or "";
+        customLuaRC = configure.customLuaRC or "";
         inherit plugins;
         inherit extraName;
       };