No mixin forwarder when ancestor is sealed

[som-snytt]

Fixes #23479

Adds check for Java sealed ancestor to needsMixinForwarder.

If it's ok to invokespecial an indirect superinterface, then the fix should be in the backend, to not add the interface parent.

[som-snytt]

Not settled on the solution, but added // scalajs: --skip.

[som-snytt]

Looking into a Scala 2 issue, JVM spec says

• Each invokespecial instruction must name one of the following:
– an instance initialization method (§2.9.1)
– a method in the current class or interface
– a method in a superclass of the current class
– a method in a direct superinterface of the current class or interface
– a method in Object

by which I understand direct superinterface to mean the forwarder won't work if not implementing the superinterface.