Add support for sealed classes (Java 17)

[rweisleder]

Java 15 will bring us JEP 360 (preview feature) which introduces sealed classes and permitted subclasses.

Currently, we are using ASM 8.0.1 which already contains experimental support for permitted subtypes. This merge request for ASM 9 contains the official support for this feature.

To do: Which API methods should be added? And should the permitted subclasses be considered as dependencies (if so, in which direction)?

[rweisleder]

Sealed classes will be part of Java 17, see JEP 409.