PolyKit is both a run-time library (provided via the PolyKit NuGet package) and a set of ready-to-compile source files (provided via the PolyKit.Embedded NuGet package) that add support for latest C# features as well as recent additions to the .NET runtime library, even in projects targeting .NET Framework or .NET Standard 2.0.
Want to know more? Here's the complete README.