Add shutdown hook to destroy the process

[Heliosmaster]

This PR adds a shutdown hook which automatically destroys the spawned process when the caller is about to quit.

Example usage: We have a babashka script which starts a bunch of (running processes). When we Ctrl-C to quit the script, we would like the processes to terminate as well.

We added an option to pass to process to disable this behavior.

[borkdude]

@Heliosmaster Seems good to me. I prefer the name without question mark (use question marks for predicate functions only: bbatsov/clojure-style-guide#182 (comment)).

Proposed name: :destroy-on-shutdown which defaults to true.

[Heliosmaster]

@borkdude sounds good. In our previous implementation it was opt-in with destroy-on-shutdown? as the option flag :-) We'll remove the question mark and force push.

[borkdude]

@Heliosmaster Documentation would also be nice.

[borkdude]

@Heliosmaster @hamann I have an example bb script here to kill all children of a process:

https://github.com/borkdude/babashka/blob/master/test/babashka/scripts/kill_child_processes.bb

Maybe it would also be useful in this context. Not sure if it warrants a different option or that we should just do this by default using this option.

[hamann]

@borkdude After playing with the proposed solution for several times we realized that just calling (.destroy proc) is not sufficient in our case because it immediately returns and let the main process exit, but the subprocess is still alive for a while, producing uncontrollable output on the terminal and then exits. So in our case we would also have to add (.waitFor proc) and maybe (.destroyForcibly proc) after some time as a last resort.
So I have the feeling that instead of implementing the shutdown logic in the process function we should give the caller control over the shutdown behaviour for example with a nasty callback function or passing an atom where the process object will be stored/referenced.

[borkdude]

OK, I'll wait with merging until we have more clarity on this.