Completion based on man page/help page and/or zsh

The current completion system is quite poor: it is impossible to generate completion for command arguments without manually writting an extern completion for any new command. Carapace-bin and the user script repository provide some pre-made extern for some commands, but the list of supported commands is really small. In any case, it can't generate completion for tools that have not been manually added.

On the other hand, fish has a python script that parses man pages to generate automatically the completion. It would be great to have it in nushell. Here is the repo that adapts the script for bash: https://github.com/nevesnunes/sh-manpage-completions

We could also maybe parse bash/zsh completion files to get more completions for free. For instance this project has a great autocomplete functionality (no idea how it's working…), it can even complete nix packages when typing nix-shell -p vim, even as you type: https://github.com/marlonrichert/zsh-autocomplete/

References

Fish script adapted to bash: https://github.com/nevesnunes/sh-manpage-completions

No implementation files, entry points, or tests are named. Start by reviewing Reedline's existing completion system, then compare the referenced fish-to-Bash man-page script and zsh completion approach. Done would require an agreed design and working automatic completion beyond manually added extern completions.