feat: Add completion evaluation to the activation of the shell on install

[ruben-arts]

This adds the pixi completion command to the activation of a shell on installation. This should make the initial setup easier.

Related to #292

[wolfv]

Would it be better to do it like rye and place a file in ~/.pixi/env.sh or so that does both these actions? That way we would only ever have to add one line to the .bashrc file, and it would be easier to remove / change.

[baszalmstra]

With #316 the original need for this PR will be removed. But should we still add this? I assume the user has to give consent?

We also have to incorporate changes from #325

[ruben-arts]

Closing this as there is no real NEED for this to make pixi work. We could revisit it later if ther're more ideas on it.