2.2.0: C# 8, NRT annotations, RollForward=Major and SourceLink

Version 2.2.0:

• Enables C# 8 and adds nullable reference types (NRT) annotations.
• Enables RollForward=Major to allow running the tool on .NET Core 3.x or later (fixes #42).
• Builds with .NET Core 3.1 SDK and SourceLink enabled.
• Re-targets tests to .NET Core 3.1 and .NET 4.8.
• Upgrades NuGet packages used for tests.

Packages are published on NuGet with SourceLink enabled.

The cross-platform global tool, dotnet-xdt, for .NET Core SDK 2.1 and later:

dotnet tool install --global dotnet-xdt --version 2.2.0

The .NET Standard 2.0 library, DotNet.Xdt can be referenced to apply XDT transforms:

dotnet add package DotNet.Xdt --version 2.2.0

A standalone dotnet-xdt.exe for .NET Framework 4.6.1 or later, downloadable under Assets above.