Naming: rewatch -> rescript

[cknitt]

As rewatch will become the default build system and cli (see #7588), we should replace the name "rewatch" by just "rescript", at least wherever it is user-facing.

[cknitt]

For example, rescript -h says

Rewatch is an alternative build system for the Rescript Compiler bsb (which uses Ninja internally). It strives to deliver consistent and faster builds in monorepo setups with multiple packages, where the default build system fails to pick up changed interfaces across multiple packages
...

🙂