Allow getting exit value from a spawned process

[KoBeWi]

Describe the project you are working on

A secret project I'll share soon 👀

Describe the problem or limitation you are having in your project

I'm using the new pipe API (godotengine/godot#89206) to spawn and read output of a process. The problem is, when the process ends, I don't know what code it returned on exist and there doesn't seem to be any way to check that.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Add some function to get exit value of a spawned process (probably based on its PID).

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

OS.execute() can already return exit code, so I think it's possible for OS.start_process() and OS.create_process() and OS.execute_with_pipe(). I think the new method could be OS.get_exit_code(pid) (and we already have OS.is_process_running(pid).

If this enhancement will not be used often, can it be worked around with a few lines of script?

Literally impossible.

Is there a reason why this should be core and not an add-on in the asset library?

It can't without tinkering with C++ OS API.