Allow abstract lazy vals, deprecate lazy vals overriding regular abstract vals

[smarter]

In Dotty, abstract vals cannot be overriden by lazy vals for soundness reasons (scala/scala3#1051). On the other hand, we allow abstract lazy vals unlike scalac. Could this limitation be lifted to ease migration ? (It's not a huge deal because one can always use an abstract def instead, but it still would be nice to have).

[smarter]

/cc @adriaanm since I think you're the resident lazy-vals-in-scalac expert :).