Scala 3. Derivation for enums. Better derivation for sealed traits

[susliko]

Addresses #199, #281

Derivation for enums

Codecs are now derived for Scala 3 enums, both simple and parametrized.
All Phobos annotations and ElementCodecConfig are respected
E.g., the following declaration is now valid:

enum Domestic derives ElementEncoder, ElementDecoder:
  @discriminator("tiger") case Cat
  @discriminator("wolf") case Dog(@text breed: String)

Derivation for sealed traits

Up to this moment sealed trait derivation required explicit derivation of instances for its children:

sealed trait Wild derives ElementEncoder, ElementDecoder
object Wild {
  @discriminator("cat") case object Tiger                    extends Wild derives ElementEncoder, ElementDecoder
  @discriminator("dog") case class Wolf(@text breed: String) extends Wild derives ElementEncoder, ElementDecoder
}

For the sake of ergonomics and consistency with enum derivation, the following declaration is now valid:

sealed trait Wild derives ElementEncoder, ElementDecoder
object Wild {
  @discriminator("cat") case object Tiger                    extends Wild
  @discriminator("dog") case class Wolf(@text breed: String) extends Wild

Instances for case object children of sealed traits are also derived now

Side changes

• fixed typo in the EncoderError message
• formatted Scala 3 files, updated .scalafmt.conf

[valentiay]

Hi! Thanks for your contribution. I'm glad to see, that enum derivation is now supported. The PR looks good to me. Going to merge and publish it in 0.21.0