Fixes #25802
markup_fmt plugin supports some HTML-like formats like Angular, Jinja,
Twig, Nunjucks or Vento, that are not supported by `deno fmt`. This PR
adds support for the extensions `njk` (Nunjucks) and `vto` (Vento).
Angular doesn't have a custom extension (it uses `html` afaik) and Jinja
and Twig are template engines written in Python and PHP respectively so
it doesn't make sense to be supported by Deno.
