Add rewatch sources and build rewatch in CI

[cknitt]

First step of #6760.

• Add rewatch sources
• Build rewatch binaries for the different platforms in CI
• Add rewatch binaries to npm package + a rewatch wrapper script that invokes the correct executable platform. (Eventually this should be integrated into the main rescript cli, or rewatch should replace the main rescript cli.)

[cristianoc]

Maybe extend the clean command too, if it's possible for rewatch to need cleaning in order to unblock the build.