add missing method implementation to JSArrayQueue

[larsrh]

This implementation was missing in JSArrayQueue. Inexplicably, Dotty complained, whereas Scala 2 didn't see a problem:

[error] -- Error: /home/lars/proj/monix/monix/monix-execution/js/src/main/scala/monix/execution/internal/collection/JSArrayQueue.scala:28:27 
[error] 28 |private[monix] final class JSArrayQueue[A] private (_size: Int, triggerEx: Int => Throwable = null)
[error]    |                           ^
[error]    |class JSArrayQueue needs to be abstract, since def size: => Int is not defined