Add rewatch as an alternative build system to eventually replace bsb

[cknitt]

• Add rewatch to compiler repo + CI + the rescript npm package (as an experimental alternative to bsb)
• Use rewatch to build the standard libraries (OCaml stdlib, Caml_*, Belt, Js)
• (Move Core into the compiler, move other stdlibs out of the compiler, ...)
• Remove bsb, do not ship any compiled stdlib artifacts with the ReScript npm package anymore (see Split rescript npm package into multiple subpackages #6183)
• Remove bsb support from reanalyze https://github.com/rescript-lang/rescript/pull/8149/files#diff-f762e3324f935368317a5ac639e27f6b323a29eef74b2aa6340db6d82c370195R137

[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.