[WISH] per file ppx flags

[gares]

It would be nice to be able to specify different ppx related flags on a per file basis.
In particular this would help implementing the per_module dune directive in a non hackish way.
As of today, dune is able to handle per module ppx directive, but is unable to generate a merlin file that actually works.

See also: ocaml/dune#1212

[trefis]

All that needed to happen on the merlin side has happened and been released on opam (thanks to @voodoos), the ball is now in dune's court.

[gares]

Thanks!!!