neovim: Add treesitter grammars

Fixes an issue with the markdown grammar not being found and adds some
extra features such as highlighting the current indentation line.

home/neovim.nix

@@ -464,6 +464,7 @@ in
       vim-astro
       vim-svelte
       typst-vim
+      nvim-treesitter.withAllGrammars
     ];
   };