Introduce an optional Scala 3 macro module, with full pickling support.

[strelec]

Hey,

I was thinking it would be good to have an optional (separate module), Scala 3 only, macro pickling module.

It would preferably function similar to uPickle, so handle default values (omit JsObject key if same as case class default, synthesize it when parsing).

The jsonFormat{1...22} methods are clumsy and have a lot of warts.