Consider generalizing onScrollElement to support EventTarget instead of HTMLElement

[liaujianjie]

Since onScrollElement is only used to mount the scroll event listener, I suggest typing the onScrollElement property on the useVirtual hook option as React.RefObject<EventTarget> as opposed to the existing React.RefObject<HTMLElement>. This enables the window or document objects to also be used as the scroll container.

I can open up a simple type change PR for this if my suggestion is within the scope of this library. :-)

How I'm using it

const parentRef = useRef<HTMLElement>(document.documentElement);
const rowVirtualizer = useVirtual({
  size: itemsToRender.length,
  parentRef,
  onScrollElement: useRef(window as any), // Casting as any to work around the narrowed type
  scrollOffsetFn: () => document.documentElement.scrollTop,
});