feat: add wrestic completions (writes to stdout)

[tippfehlr]

This adds wrestic completions.

This is the standard (as observed by me, at least).
One example is Starship.
It would also make it a lot easier to include (all) completions in packages.

I left wrestic --generate in place, but haven’t looked at it further.

[alvaro17f]

Finally I've introduced this feature removing the --generate subcommand, so it will be agnostic to the system or path.
Users now will have to manually configure completions, but I'll be improving docs to take care of this.