Auto-generate zshell script to perform completions for a CLI

[jdegoes]

See here for a tutorial, here for the reference, and #2 for the corresponding Bash ticket for more details.

[h-ayat]

Interesting feature, I'd like to work on it.

[jdegoes]

@h-ayat If you take a look at Fury, a Scala build tool, you can see an example of how this feature is used to great effect.

We have several choices of how to do this one. First off, the auto-completions could be "dumb" and generated from introspecting the Command. Second, they could be "smart" and the shell script could specify running the CLI App in a special mode to generate the auto-completions (e.g. --auto-completion "...").

The CLI App has enough information to do context-sensitive completions. Although this would have to somehow integrate with #5, because if a user types --verb, then we have to identify which option starts with --verb to produce the completion --verbose.

You may be able to find a person to pair with you on this feature in the Discord chat room. In any case, if you have any questions or need a hand, let me know!