Implement java.util.concurrent.ScheduledExecutorService

[lolgab]

java.util.concurrent.ScheduledExecutorService is a Java API for scheduling tasks that can be used to abstract over Scala Native scheduler implementations.
ZIO JVM uses java.util.concurrent.ScheduledExecutorService to implement its default scheduler: https://github.com/zio/zio/blob/881beb92b36b7e7c32ee19115c5c6d3a75bc59a9/core/jvm/src/main/scala/zio/Scheduler.scala#L22
Currently ZIO Native uses scala-native-loop's Timer API. This is suboptimal since it couples ZIO with scala-native-loop.
If both scala-native-loop and epollcat implement this API, it can be used by ZIO and other runtimes leave the runtime decision to link-time.
Companion issue: armanbilge/epollcat#82