Feature: JsonDiscriminator

[goshacodes]

Proposed feature for Scala 3:

1. Derive JsonReader for sum types with provided implicit JsonDiscriminator.
2. Derive JsonWriter with discriminator if JsonDiscriminator for type is in scope

trait JsonDiscriminator[A, B]:
  def choose: A => B

object JsonDiscriminator:
  def by[A, B](f: A => B): JsonDiscriminator[A, B] = new JsonDiscriminator[A, B]:
    override def choose: A => B = f
   

Example:

enum Choice(val discriminator: Int) derives JsonReader, JsonObjectWriter:
  case First(first: String) extends Choice(0)
  case Second(second: Int) extends Choice(1)

object Choice:
  given JsonDiscriminator[Choice, Int] = JsonDiscriminator.by(_.discriminator)
  


(First("123"): Choice).asJson === {"discriminator": 0, "first": "123"}

{"discriminator": 0, "first": "123"}.jsonAs[Choice] === First("123")