std.process.Child: detached child spawning

[antosikv]

Allow spawning a detached std.process.Child.

Tested manually on linux and windows, both seem to work and stay alive after closing the terminal/console. Simple test included but any suggestions for a better one definitely welcome (especially for windows).

Closes #19205

[antosikv]

It might also be a good idea to add some sort of deinit function for when you have no intention of killing or waiting to properly dispose of any file descriptors/handles.

Also, any errors reported from the child that happen between fork and exec are currently only read after waiting or killing. Would it be a good idea to block in spawn until the child gets all the way to exec and return any errors up to that point immediately from spawn (it even has the correct signature for it so no breaking changes)? It would at least give you an opportunity to try again before you go do something else and in the case of detached processes it might be your only chance to know that the process didn't actually start doing anything.