The handler for SemanticsAction.scrollDown.
This is the semantic equivalent of a user moving their finger across the screen from top to bottom. It should be recognized by controls that are vertically scrollable.
VoiceOver users on iOS can trigger this action by swiping down with three fingers. TalkBack users on Android can trigger this action by swiping left and then right in one motion path. On Android, onScrollDown and onScrollRight share the same gesture. Therefore, only on of them should be provided.
final VoidCallback? onScrollDown;