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

As described in , 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.