Improve error message for "unknown commands" for well-known sub commands

[MylesBorins]

Describe the feature or problem you’d like to solve

As a long time hub user I have tried to run gh fork a number of times to be reminded that I have to run gh repo fork.

I was informed that i could make an alias to avoid this error in the future by @mislav.

gh alias set fork "repo fork"

Would be great to see if we could impart similar wisdom automatically as part of the error message.

Proposed solution

When a unknown command is provided but it is a well known subcommand such as fork we could provide a slightly more actionable error message such as

$ gh fork
unknown command "fork" for "gh", did you mean to run "gh repo fork"?

You can create a an alias for this command with the following command
`gh alias set fork "repo fork"`

Additional context

Add any other context like screenshots or mockups are helpful, if applicable.

[g14a]

@mislav is it okay if I pick this up? Personally I can relate to this issue because gh has become vast and complex recently with so many commands and subcommands, so one tends to get confused if they're new to the tool.

I'd like to know what sort of improvement can be brought here..

[mislav]

Sure, go right ahead! 🚀

I think how this could work is: if there is no command (and no aliases nor extensions) matching gh foo, what gh could do is iterate all sub-commands and reporting matching ones:

$ gh foo
unknown command `gh foo`
did you mean:
  gh repo foo
  gh issue foo
  gh ssh-key foo

The formatting doesn't have to be exactly like the above; it was only for illustration purposes. As a real-world example, today Cobra has this behavior where it detect mistypings of commands:

$ gh issu
unknown command "issu" for "gh"

Did you mean this?
        issue

As a special case, if there is only one match across all subcommands, then the error message could be something like @MylesBorins suggested where the error message tells you that could add your own alias for this in the future. That would work well for verbs like clone and fork because it's very unlikely that they will have matches elsewhere other than under gh repo.

[jf-delineate]

@g14a are you actively progressing this? I'm happy to pick up myself if needed.

[g14a]

@jf-delineate please do go ahead. I got caught up with other stuff over the months. Good luck :)