csharp: Centralize more options like nullable warnings and treat warnings as errors to keep the build warning-free

[CurtHagenlocher]

What feature or improvement would you like to see?

Now that the C# build is warnings-free, we should enable TreatWarningsAsErrors to help keep it that way.

Options like TreatWarningsAsErrors, Nullable and ImplicitUsings should have centralized default values in Directory.Build.props instead of having to be set in each individual project file.