.NET build support package. Configures the following build behaviour: * Set C# language version to C# 9 * Enable Nullable Reference types. Uses the Nullable NuGet package to back-reference NRT annotation attributes for older .NET TFMs * Generates portable PDB-files for debugging in Debug configuration. * Add targets that trims the FileVersion assembly attribute to a 3-component version number * Add targets that trims the AssemblyVersion assembly attribute to a 3-component version number and adds the BuildId property (if configured) as the 4th version component.