[wip] implement "detaching" from shell to start a background process [skip ci]

[wolfv]

This uses the daemonize crate to detach the process and effectively daemonize it.
That is kinda cool for long-running processes (e.g. if I run jupyterlab).

There is one big issue: for some reason, creating the activation subprocess does not seem to work and I haven't figured out why yet.

Other TODOs:

• store PID file in .pixi folder
• list running tasks
• kill running tasks
• look at stdout of running tasks

[ruben-arts]

What is the status of this PR?

[hadim]

I gave a try to it at #682