Called, as needed, to build list item widgets.
List items are only built when they're scrolled into view.
The AnimatedListItemBuilder index parameter indicates the item's position in the list. The value of the index parameter will be between 0 and initialItemCount plus the total number of items that have been inserted with AnimatedListState.insertItem and less the total number of items that have been removed with AnimatedListState.removeItem.
Implementations of this callback should assume that AnimatedListState.removeItem removes an item immediately.
final AnimatedListItemBuilder itemBuilder