feat: add specialized zsh task completion

[wolfv]

This adds some completion for arguments after the task. For example

pixi run git chec<TAB>
# completes to `checkout`
pixi run foo ./my-fi<TAB>
# completes to a filename (e.g. `./my-file.txt`)

Only works with zsh for now but I think we can have a similar approach on bash.

Does not (yet) complete the binaries found in the pixi env (e.g. $PIXI_ENV/bin/<binary-name> could also be a nice completion opportunity.

Does not take into account flags like -e ENV.

[pavelzw]

xref #372

do you think such an approach would work for fish as well?