Add rewatch as an alternative build system to eventually replace bsb

[cknitt]

1. Add rewatch to compiler repo + CI + the rescript npm package (as an experimental alternative to bsb)
2. Use rewatch to build the standard libraries (OCaml stdlib, Caml_*, Belt, Js)
3. (Move Core into the compiler, move other stdlibs out of the compiler, ...)
4. Remove bsb, do not ship any compiled stdlib artifacts with the ReScript npm package anymore (see Split rescript npm package into multiple subpackages #6183)

[nojaf]

Is it the plan to make rewatch available from the node_modules/.bin folder?
It is not yet the case with 12.0.0-alpha.1.

[cknitt]

Yes, that would be the plan. Seems this was lost in a refactoring where the path to the rewatch wrapper script in package.json was not updated.

[nojaf]

This works for me in 12.0.0-alpha.2, thanks!

[cknitt]

rewatch is now included in v12, bsb will be removed in a future version. Core is now included in the compiler repo as the new stdlib.

Other items still in progress.