New function to return console command line as string

[link0ff]

Should I create a separate issue for this?

Yes, please.

Originally posted by @dgutov in https://github.com/nonsequitur/inf-ruby/issues/121#issuecomment-726808836

Such a function could return the command line without executing it. This will help to use the console command in ob-ruby.el.

A prototype patch also wouldn't hurt.

Sorry, I have no idea how to refactor the existing functions.