Add `std::process::Command`-like API for spawning processes

[Ottatop]

Let's be real here: callbacks are good for some things and not for others. Signals being one of the "some things" and the current Process::spawn API being the "others".

I'd like to implement an API mirroring Rusts Command for spawning processes. Possibly also on the Lua side. This should make it less tedious to do things like wait for spawned processes to exit.