Scala 3 caveats

[ghostdogpr]

A few other things I discovered and that you might want to mention somewhere: Scala 3 typeclass derivation doesn't support a few things that Magnolia supported with Scala 2:

• value classes
• nested sealed traits

Opaque types also don't have mirrors. And I think default values can't be obtained from the macro yet.

[adamw]

I suppose a good idea here would be to move all of the magnolia-scala2 tests into magnolia-scala3, and see which ones fail. Mark them as such, but keep them around as a good documentation for further work.