Ability to format non-OCaml files

Desired Behavior

dune build @fmt can format anything with a dialect defined, but dialect requires the files to be OCaml-like (have an implementation and interface and compile into OCaml). It would be nice if we could define formatters for non-OCaml files. For example, Dune internally has a formatter for dune files, but I don't think it's possible for users to define similar formatters.

Examples of things we want to format:

• atdgen files
• random Python scripts in our build system
• Various other sexp files we use in our builds

Example

Something like:

; dune-project
(formatter
  (extension py)
  (format python3 -m yapf -))

.. and then dune build @fmt would run the equivalent of python3 -m yapf < python_file.py for each Python file.

Start by reading the formatter behavior behind dune build @fmt and the existing formatter definition for Dune files; the issue also gives a proposed (formatter ...) entry in dune-project. Done means users can register formatters for non-OCaml files, including the shown Python command, and have them run for matching files through @fmt.