ZIO Streams 2.0: implement when/whenZIO

The ZIO Streams 1.0 API defines when, whenM (deprecated) and whenZIO. These are not yet defined on the new zio.streams.experimental.ZStream.

The goal is to have all the methods of the old ZStream trait and companion object starting with when (when, whenCase, whenM, whenCaseM, whenZIO, whenCaseZIO defined in the new ZStream too (including the deprecated ones!), to avoid breaking the API.