Show help for custom subcommands by default if parent command is not defined

Related problem

Imagine I want to define new commands that are actually subcommands for non-existing command

def 'service start' () {}

def 'service stop' () {}

Calls to service start, service stop will work fine, but call to service will fail with No such file.

In order to provide some guide when calling service I can define parent command with next code:

def service () {
  help service
}

And with that after calling service I'll get the help for subcommands

Describe the solution you'd like

Provide default parent command that will show help message if some of subcommands were provided. In case the parent command is defined, use it instead of default.

Describe alternatives you've considered

I can implement that manually every time I define subcommands, but this will be tedious.

Additional context and details

No response

Start by tracing Nushell's custom subcommand resolution for calls such as service start and the existing help service behavior. Define the fallback behavior when service has subcommands but no parent command, while preserving an explicitly defined parent command, and verify both cases with focused command-dispatch tests.