VariableSizeList: Dynamic Row Heights

Sometimes rows are not uniform: a chat list mixes short and long messages, or a feed has cards of varying content. FixedSizeList cannot handle this because it assumes one constant itemSize.

For these cases react-window offers a list type that lets each item declare its own size.

VariableSizeList works like FixedSizeList but accepts a per-item size. You import it instead and provide a function that returns the size for any given index.

Everything else, the children render function and the critical style prop, stays the same, so the migration is small.