Migrate LuhnCheck and LuhnLibrary to .NET Core

[MylesFTOP]

These have already been upgrade from .NET Framework 4.6.1 to 4.7.2, but they need to migrated to .NET Core (currently expected to target 3.1, in line with LuhnLibrary.Tests). This will probably be done first with LuhnLibrary by renaming it and then creating a new .NET Core project "behind" it (i.e. with it inheriting from the .NET Core project), and then gradually moving code over to the new project until the .NET Framework version can be dropped.

This is ideally done after measuring code coverage in issue #20.