Fix supporting modifying sealed abstract classes in Scala 3

[KacperFKorban]

This PR allows for sealed abstract classes to be modified in Scala 3.

All children of sealed abstract classes are known at compile-time, just like the children of sealed traits, so they should be supported as well. (They worked for Scala 2.13, there was just no test for it)

I tried adding this test for all Scala versions, but it seemed to fail for Scala 2.11 with:

[error] quicklens/quicklens/src/test/scala/com/softwaremill/quicklens/ModitySealedAbstractClass.scala:9:18: Could not find subclasses of sealed trait class Variance.
[error] You might need to ensure that it gets compiled before this invocation.
[error] See also: <https://issues.scala-lang.org/browse/SI-7046>.
[error]     invInt.modify(_.typ).setTo(Type("Long")) should be(invLong)
[error]                  ^
[error] knownDirectSubclasses of Variance observed before subclass Covariance registered
[error] knownDirectSubclasses of Variance observed before subclass Contravariance registered
[error] knownDirectSubclasses of Variance observed before subclass Invariance registered
[error] four errors found

[adamw]

Thanks! :)