Support animated resizing within VariableSizeList

[gmattie]

The <VariableSizeList> component supports item resizing via the resetAfterIndex(index) function. However, while this functionality is great for immediately resized item renderers, it does not support smooth animation resizing.

Animated resizing is available in another virtual list library, React Virtuoso, but are there plans to include similar functionality to react-window (or react-virtualized)?

Originally posted by @fonty422 in #92 (comment)

I know this is closed for now, but what about an animated expanding div. Not just something with 2 hard coded values. I have a need to use the material-ui ExpansionPanel, but implementing this leaves me with a panel that expands but doesn't displace the rest of the list.