[shell] Add option to spawn command in process group

this is basically the fix to issues like pyinstaller apps not being killed because pyinstaller wraps the actual app in a thin parent process that doesn't clean up after itself.

v1's command code was a bit too much for me to find motivation tbh but in this plugin it should be fairly straight forward.

Since may be use cases where this is not desirable, it will be a simple boolean option.

The implementation should use https://github.com/watchexec/process-wrap, the successor of command-group.

p.s. just opening this issue so i don't forget about it until next week, not asking the team to work on this (however, if a community member wants to work on this then i'd be quite happy about it)