This updates the cppwinrt port to use the latest
Microsoft.Windows.CppWinRT package from NuGet. Includes clean up of the
portfile for modern best practices.
This also cleans up the cmake integration:
* Uses `cppwinrt_FOUND` instead of the nonstandard `CppWinRT_FOUND`
* Provides `CPPWINRT_TOOL` for being able to generate C++/WinRT headers
from .winmd files as custom commands
