feat: add a optional separatorBuilder

This awesome library is sadly missing a way of simply adding spacing or other widgets inbetween the lists widgets, like Listviews separatorBuilder argument.

Start by locating the library's main animated list widget and its existing item-builder API, then compare the requested behavior with Flutter's ListView separatorBuilder argument. Define how optional separators should appear between items and how they should behave during list animations; done means spacing or other widgets can be supplied between list items without breaking the existing behavior.

Written by the indexing model from the issue text.