maintain completion functions for the CLI

[tshu-w]

🚀 Feature

Current, jsonargparse use argcomplete to provide completion which doesn't support zsh well. However it's hard to use without completion in terminal as there are many options.

Additional context

I tried to make it by myself but failed https://www.reddit.com/r/zsh/comments/r36fxg/completion_gnu_generic_for_subcommand/

cc @Borda