For some reason `SIGTTIN` sometimes gets sent to the process group,
causing it to stop when run from a terminal. This solves that issue by
putting the shell in a new session + progress group.

This allows removal of a workaround of using `exit 0;` to restore
handling of ctrl-c after exit. In testing this appears to no longer be
necessary.

Closes #27716

Release Notes:

- Fixed Zed sometimes becoming a stopped background process when run
from a terminal.