Switch from Newtonsoft.Json to System.Text.Json

.NET Core 3.0 came with a new built-in JSON serialization library System.Text.Json which probably becomes the standard JSON library for .NET. It offers a better performance while allocating less memory compared to Newtonsoft.Json because it is using new technologies such as Span<T>.

For users of .NET Core 3.x this also has the nice benefit that the JSON implementation is directly included in the .NET Core framework which solves possible version conflicts.