appendPage method
Appends newItems
to the previously loaded ones and replaces
the next page's key.
Implementation
void appendPage(List<ItemType> newItems, PageKeyType? nextPageKey) {
final previousItems = value.itemList ?? [];
final itemList = previousItems + newItems;
value = PagingState<PageKeyType, ItemType>(
itemList: itemList,
error: null,
nextPageKey: nextPageKey,
);
}