[feature request] Add ability to launch a detached process

[andyburke]

Eg:

long-running-process &

This would require passing the detached option to spawn, and there is the caveat:

When using the detached option to start a long-running process, the process will not stay running in the background after the parent exits unless it is provided with a stdio configuration that is not connected to the parent. If the parent's stdio is inherited, the child will remain attached to the controlling terminal.