Sum types are sealed traits, sealed classes and non-singleton enums.
Since they have one or more subtypes, we need to disambiguate between them somehow.

The default in Tupson is to use the @type property.
Its value is the simple type name of class or enum case.
This makes JSON independent of scala/java package and it is more readable.

Example:

      enum Color derives JsonRW:
        case Hex(num: String)
        case Yellow

      val color = Color.Hex("FFF")
      
      println(color.toJson)
      // { "@type":"Hex", "num":"FFF" }
      

          @discriminator("myOtherKey")
          enum Color derives JsonRW ...

          println(color.toJson)
          // { "myOtherKey":"Hex", "num":"FFF" }