Proposal: A protocol watch processes can implement

Hi guys

Because I was unhappy about the situation with watch-processes, or mainly their inability to coordinate with other watch-processes, I recently started my hobby project [task-graph-protocol](https://github.com/VanCoding/task-graph-protocol).

The main idea I had was a simple protocol that watch-processes could implement to communicate with the task-runner, so that the task runner knows when a task is completed even if the process does not exit, and the task knows when to re-start.

I haven't heard of `wireit` before and because this whole protocol doesn't make sense without a task-runner implementing it, I also implemented a simple task runner for it. But `wireit` of course is much more mature, and it would be much simpler to integrate the protocol into it than to extend my task runner to be as powerful as `wireit`.

So my proposal is to integrate the `task-graph-protocol` or the idea of it into `wireit`. And if you're really going to do it, we could try to bring all the tools like typescript, webpack, etc on board. That's also much easier if the protocol is backed by a major task runner.

What do you think? :)

Btw, I have already implemented the protocol for TypeScript, it's included in the repository.