this has two main advantages:
- it makes it clear that the shells are mutually exclusive
- it allows us to extend the command with shell-specific options in the future if necessary
as a happy accident, it also adds support for `elvish` and `powershell`.
for backwards compatibility, this also keeps the existing options as hidden flags.
i am not super happy with how the new help looks; the instructions for setting up the shell are
squished together and IMO a little harder to read. i'm open to suggestions.
