Implement shell-hook

[wolfv]

Problem description

There are a few use-cases where having a traditional "activation script" could be useful. It could be a nice addition to have a (hidden?) shell-hook command that prints the activation script to stdout.

A potential pixi-docker project could use this to source the environment even without pixi being present (this script could be written as a step in the docker build and sourced by the docker entrypoint).

cc @vsoch

[pditommaso]

This would be definitively very useful. We are currently using micromamba-docker to assemble container images from Conda packages via Wave.

We would love to use Pixi in place of Mamba once this capability is going to be available.