Create melange-ppx as standalone ppx rewriter

[jchavarri]

There are a number of syntax transformations that Melange does internally, for example:

• Pipe first operator |.
• Object method selector ## (and another less known #=)
• Processing of bs attrs
• ...

The main entry point is bs_builtin_ppx.ml.

For merlin to work properly, we currently run melc as ppx from dune (see https://github.com/ocaml/dune/pull/6476). Ideally in the future there should be a melange-ppx that allows to do this processing like with any other ppx, so that merlin doesn't require any explicit hacks to work with Melange.

[anmonteiro]

this was done in #534