Support OCaml 5.2+ functor parameter syntax

Versions

• mlx: 0.11
• OCaml: 5.5.0
• ocamlformat-mlx: 0.28.1.2

Problem

mlx-pp rejects the newer functor type syntax accepted by OCaml 5.2+ and emitted by ocamlformat-mlx when formatting for current OCaml versions.

Reproduction

mlx-pp -print-ml <<'MLX'
module type T = sig
  type t
end

module Fresh : (X : T) -> sig
  type t
end =
functor (X : T) -> struct
  type t = X.t
end
MLX

Actual result:

File "*stdin*", line 5, characters 18-19:
Error: Syntax error: ')' expected
File "*stdin*", line 5, characters 15-16:
  This '(' might be unmatched

The same declaration is accepted by the OCaml 5.5 toplevel.

Context

This appeared while converting a Reason functor to MLX. ocamlformat-mlx produced the syntax above, but the Dune MLX preprocessor could not parse the formatted file.

Start by running the provided reproduction through the mlx-pp preprocessor and tracing its parser handling of module and functor type declarations. Inspect the parser entry points involved in the reported syntax error; done means the OCaml 5.2+ declaration is accepted and the Dune MLX preprocessing path has a regression test for it.