Rewatch - Run build when there's a running watcher

[DZakh]

With Rescript Legacy, it's possible to run a script containing a build command while there is a running watcher. With Rewatch, it requires you to stop the voucher to run the script. It would be nice to be able to have the same developer experience as before.

[DZakh]

Potentially just exit with 0 when we see that there is a running watcher and the project is correctly compiled, so it can pass through to the following commands in the script.

Could not start ReScript build: A ReScript build is already running. The process ID (PID) is 15354

In my case, during development, I run pnpm test from time to time, and it contains a rescript build which breaks when there is a running watcher now.