Allow using a PPX without passing the dependency on

[mlms13]

The problem:

Bastet, Relude, and presumably other libraries make use of bisect_ppx during development/CI to collect test coverage stats. In an ideal world, users of those libraries wouldn't also have to depend on bisect_ppx, because it's a dev-only tool that at various points has caused pain when building on Windows, with the Rescript compiler, etc. Currently, I don't believe it's possible to make it a devDependency, because the ppx-flagsfield in our bsconfig will try to run during every build, and it will fail in this case if it doesn't find bisect_ppx.

Maybe a potential solution:

@aantron had proposed adding a "dev" version of the ppx-flags field to bsconfig.json. This could theoretically allow users of Relude to build a non-development version of the library that skips the bisect_ppx step. There was even a Bucklescript PR that attempted to fix this, but it was never merged.

Related info:

• Dune support for bisect_ppx

[anmonteiro]

This is related to #192 and #92

[anmonteiro]

Same reasoning as #92, we'll get dune profiles for free with the deeper dune integration.