fix child processes becoming zombies in dev

[qw-in]

fixes #317

TL;DR I was using volta, which spawns node in a child process & it wasn't getting cleaned up.

This pr attempts to address that by always cleaning up the process & all sub-processes. To do so I lifted some code from go-cmd.

I didn't try very hard on the naming/factoring - happy to change it to whatever y'all prefer

Not sure if it could be used in more places too - noticed something similar in client already:

https://github.com/sst/ion/blob/11e5f33cb057ac36615073e055893c65b1c97b69/pkg/server/client.go#L41