Support gemini mime inference

[Tim-ats-d]

Add support for .gmi file mime inference in Mehari.from_filename.

[Tim-ats-d]

Inferring gemini mime type from files with extension .gmi would make Mehari.from_filename semantics too complex. Adding an optional lang parameter used only in the case of a file containing Gemtext is questionable. It is better to implement a wrapper around this function on the user side, e.g.

let guess_from_filename ?lang ?charset fname =
  if String.ends_with ~suffix:".gmi" fname then
    Mehari.gemini ?lang ?charset ()
  else
    Mehari.from_filename ?charset fname