Add Shell Completion Scripts

[tomassliz]

It would be great to provide shell completion scripts which allow you to press the tab key on your keyboard to autocomplete commands and arguments when typing a mint command.

One approach how to achieving this is to generate them. Installation instructions for different shells (bash, zsh, fish) and the underlying implementation is available in the swift-argument-parser repo.

[yonaskolb]

I think moving over to swift argument parser is probably a good idea, which also makes this easier, in terms of implementation and documentation.