Pls add support new JsonAttributes for .NET Core 3.0 Json Serializer

[HeikoWr]

Hi,

.NET Core 3.0 brings a new Json Serializer (System.Text.Json.Serialization). New attribute names for the data members came along with this. Please add 'JsonPropertyName' to the selection box (C#).
Change class IndexViewModel:

And class CSharpCodeWriter:

Thank you :-)