Handle Merlin's PPX phase cache

[pitag-ha]

Hello @rgrinberg,

As you know, I've recently implemented a simple cache for Merlin's PPX phase. That cache needs to be explicitly enabled via the configuration.

Desired Behavior

It would be great if ocaml-lsp could enable the PPX phase cache iff the project doesn't use PPX dependencies (i.e. no preprocessor_deps field). So, iiuc, we need to distinguish two cases:

1. If the project doesn't have a .merlin-file, then ocaml-lsp needs to find out via a dune-rpc call whether the project uses PPX dependencies. If it doesn't, then ocaml-lsp should set the new use_ppx_cache field of config.merlin to true.

2. If the project has a .merlin-file, then the user (e.g. the user's build system, such as jenga) will take care of Merlin's config and ocaml-lsp doesn't need to do anything in particular (apart from updating the vendored Merlin).

Is that right? Does it sound good to you if we implement this?