feat: add shell-hook command

[orhun]

closes #410

This is a hidden command - let me know where should we document this!

[orhun]

Hmm, I'm think we can also add tests for this if needed.

[ruben-arts]

A test would be much appreciated! I'm not sure we need to hide it from the help.

[wolfv]

Looks good!

[orhun]

A test would be much appreciated!

Added in 5c65ccf

I'm not sure we need to hide it from the help.

Added about text in b86a979 in case we decide to make it visible in the future.

[ruben-arts]

He it would be awesome to have this added to the examples/docker-build project.

Here it would be nice to install something from a pixi.toml (pixi itself might be a weird example).
Then write the pixi shell-hook > pixi-activation.sh.
Then throw away the initial install image and only copy the .sh file and the environment to be able to activate it in the docker image without having pixi in the docker image.

I guess you can use docker multistage for this

[wolfv]

I think we also need to get rid of the shebang because that wouldn't work on a powershlel script on Linux etc

[orhun]

Removed shebang in 9591a1e