Consider merging WindowVirtualizer component into Virtualizer

[inokawa]

The logic of WindowVirtualizer is stable and close enough to Virtualizer. It might be better to merge them and define a new prop like scroll: ScrollHandlerInterface/resize: ResizeHandlerInterface, to support more advanced usecases.

[marioparaschiv]

The resize handler prop would be amazing for an implementation of stickToBottom. I am currently using my own ResizeObserver to achieve this but it impacts performance a ton since virtua already has its own.