neovim: add vale & ltex-lsp

outputs/pkgs/ook-vim/config/plugins/languages/markdown.nix

@@ -2,9 +2,19 @@
   vim = {
     languages.markdown = {
       enable = true;
+      vale.enable = true;
+      ltex.enable = true;
       extensions = {
         render-markdown-nvim = {
           enable = true;
+          setupOpts = {
+            heading = {
+              border = true;
+              width = "block";
+              left_pad = 3;
+              right_pad = 4;
+            };
+          };
         };
       };
     };