Split junit-quickcheck-core into -api, -junit4, and -junit5 artifacts

[vlsi]

As of now, junit-quickcheck-core adds junit:4.12 transitive dependency, which is sad since it exposes classes like org.junit.Assert to the junit-quickcheck consumer code.

What do you think if there were an API artifact and relevant engine artifacts?

In other words, API would contain the classes that are needed to create junit-quickcheck tests (e.g. annotations, SourceOfRandomness). And the engine could be junit4 or junit5 based (e.g. different artifacts).