add support to granular control over which items can be combined with within the list

[ArsSirek]

add support to granular control over which items can be combined with within the list

[ArsSirek]

Here is the example usage of the API suggested in the pull request

        const isCombineAllowedForItem = (combineWith: DraggableDescriptor) => {
            const treeItem = tree.items[combineWith.id];
            return treeItem && treeItem.data && treeItem.data.item.isCluster;
        }

        return (
            <Tree
                tree={tree}
                isNestingEnabled={true}
                isDragEnabled={true}
                offsetPerLevel={22}
                renderItem={renderItem}
                onExpand={onExpand}
                onCollapse={onCollapse}
                onDragEnd={onDragEnd}
                onDestinationChange={onDestinationChange}
                isCombineAllowedForItem={isCombineAllowedForItem}
            />
        )

since we have the whole tree that have all items already indexed in the .items field we can easily get and check conditions on per Item level