Initial support for metaocaml

[hhugo]

This PR is the result of a quick experiment to add support for metaocaml.
You should feel free to take over it and push it through.

Fix #1211

This PR

• The parser rules are already present (here and upstream)
• This PR patches lexers to emit metaocaml tokens if requested (inspired from MetaOCaml parsing mode ocaml/ocaml#12470)
• The metaocaml syntax is preserved. The corresponding extension points are not automatically converted to metaocaml syntax.
• This PR introduce a new config flag to switch metaocaml mode on

Testing

I've not added tests yet.

This PR was manually tested with the following example

$ cat meta.ml

   let rec spower n x =
      if n = 0 then .<1>.
      else if n mod 2 = 0 then .<square .~(spower (n/2) x)>.
      else .<.~x * .~(spower (n-1) x)>.
    (* val spower : int -> int code -> int code = <fun> *)

$ dune exec ocamlformat -- --metaocaml meta.ml

let rec spower n x =                   
  if n = 0 then .< 1 >.
  else if n mod 2 = 0 then .< square .~(spower (n / 2) x) >.
  else .< .~x * .~(spower (n - 1) x) >.
(* val spower : int -> int code -> int code = <fun> *)