Fragment support

[davesnx]

JSX has a nice feature called fragments (<></>), which expresses the idea of a list of elements without a parent.

https://reasonml.github.io/reason-react/docs/en/jsx#fragment

I read that we don't want to bloat the syntax here #16 (comment), but I would like to propose it and see if the change isn't too invasive by implementing it myself

[andreypopp]

The annoying part is to modify all three parsers:

1. mlx — fork of standard OCaml parser
2. ocamlmerlin-mlx — fork of Merlin parser (more recovery)
3. ocamlformat-mlx — fork of ocamlformat parser (more CST-ish info, useful for formatting)