home: vim: add 'nvim-treesitter'

IMHO, this is *very* colorful, I might not end up using the highlighting
after all.

Let's see if I get used to it after a little while.

home/vim/default.nix

@@ -66,6 +66,7 @@ in
       lightline-lsp
       lsp_lines-nvim # Show diagnostics *over* regions
       null-ls-nvim # LSP integration for linters and formatters
+      (nvim-treesitter.withPlugins (_: pkgs.tree-sitter.allGrammars)) # Better highlighting
       plenary-nvim # 'null-ls' dependency
     ];