POSIX Spawn APIs

[GeorgeLyon]

Is anyone actively working on this? I added just-enough-for-my-use-case API in my own project here: https://github.com/GeorgeLyon/Shwift/blob/main/Sources/Shell/Support/Posix%20Spawn.swift but I'd be happy to flesh it out and contribute.

Maybe this should wait on #20, but I can also probably just add the ProcessID type the same way it is proposed in that PR.

[milseman]

Definitely spin off anything you need from #20! That's a bag of related types and it's not surprising that some of them are related to other types as well :-).