Add tests and then official support for Scala 3 macros handling Scala 2.13 code and vice-versa

[MateuszKubuszok]

Once support for features currently supported in 2.13 is provided for Scala 3, add tests and later implementations verifying work with Scala 3 specific features:

• cross-compilations with 2.13
  • Scala 2.13 with -Ytasty-reader should:
    • derive code for case classes compiled with Scala 3
      • including code using default values
    • derive code for sealed traits compiled with Scala 3
    • derive code for enums derives compiled with Scala 3 (parameterless case has is val instead of case object)
    • derive code for Scala 3 class with @BeanProperty (annotation changed behavior in Scala 3)
  • Scala 3 should:
    • derive code for case classes compiled with Scala 2.13
      • including code using default values
    • derive code for sealed traits compiled with Scala 2.13
    • derive code for Scala 2.13 class with @BeanProperty (annotation changed behavior in Scala 3)
• derive Scala 3 specific things:
  • opaque types with extension methods (Scala 3 macros see extension methods on list of methods!)
  • optionally deriving code for union types - provided that cases similar to Typeable instance not being discovered on generic type typetest scala/scala3#12668 will be solved and generated matches would be safe, without the type test for Type cannot be checked at runtime issue

[MateuszKubuszok]

It seems that cross-compilation 2.13 and 3 despite being advertised as working ("sbt sandwich") still does NOT work (sbt/sbt#7405)

[MateuszKubuszok]

Removed from 0.8.0 milestone, as this has a low priority and I am unable to add these test until sbt-projectmatrix fix "sbt sandwich".