Command-line-api: Improve dotnet-suggest auto registration

[jeredm]

• [ ] Improve user experience when the user does not have dotnet-suggest installed.
• [x] Remove explicit registration

[jonsequitur]

The approach discussed during the hackathon was that if dotnet-suggest is not installed, we will detect that on configuration of the parser and set a flag that can be read by the user when help is requested. It should be reasonably attention-grabbing in most cases if there's a line of red after help output something like:

To enable tab suggestions, run dotnet tool install -g dotnet-suggest.
To disable this message, run your-command [hide-dotnet-suggest-message]`