Add Source Link support

[marcominerva]

TinyHelpers, TinyHelpers.AspNetCore and TinyHelpers.EntityFrameworkCore projects must support Source Link, as described here: https://devblogs.microsoft.com/dotnet/producing-packages-with-source-link.

At https://github.com/ErikEJ/SqlClientExtensions/blob/main/src/SqlClientExtensions/SqlClientExtensions.csproj you can find a real example of a library with Source Link enabled.