Expose `_state?.pageController?.position.activity?.isScrolling` on `CarouselController`

[ChopinDavid]

It would be useful to be able to check whether the pageController of our CarouselController is currently scrolling. As for our use case, my team is using CarouselController.nextPage and CarouselController.previousPage to animate between carousel items. We do not want to allow the user to trigger this method multiple times while the pageController is scrolling, i.e. by spam tapping the "next" or "previous" buttons. Doing so creates an ugly "stuttering" effect while the user spam taps.