Evaluate System.CommandLine for .NET Core branch of console apps

[thargy]

Microsoft recently open-sourced the System.CommandLine library, as used by the dotnet CLI. It is aiming to be the standard way for parsing command lines, and supporting features such as autocompletion in multiple cross platform shells.

To that end we should evaluate it as a replacement where we currently parse commandlines in the core libraries, as they are migrated to .NET Core.