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