Make it easier to diagnose why dotnet-suggest does not run the executable

[KalleOlaviNiemitalo]

As described in #1588 (comment), if you register a command-line executable in one directory and then try to use tab completion via dotnet suggest on an identically-named executable in another directory, it won't work unless you register that executable separately. It is not obvious how to diagnose and fix such a problem. This could be improved in the some of the following ways:

• dotnet-suggest diagnose EXECUTABLE-OR-COMMAND to describe whether EXECUTABLE has been registered, or whether the executable that COMMAND would run has been registered. Unlike dotnet-suggest get, this should Do What I Mean and not expect any -- syntax.
• dotnet-suggest list --verbose to list the full path of each registered command.
• dotnet-suggest list --help to tell the user about the ~/.dotnet-suggest-registration.txt file.

[KalleOlaviNiemitalo]

I imagine you could also have a per-shell key binding that runs dotnet-suggest get with an extra option that makes it display its own command line, whether and how the command has been registered, and how it would execute the command. This feature would be harder for users to discover, though; and it might conflict with a user-defined key binding.