Add scrollRef prop to Vue Virtualizer

[inokawa]

It's necessary for complex markup.
Just port React implementation.

[billouboq]

is scrollRef working on react ? did not found the implementation and was not being able to make it work with a list that has header before the actual list and scroll is done by some parent view

[inokawa]

@billouboq
React's implementation is here.

virtua/src/react/Virtualizer.tsx

Lines 262 to 267 in d6faef0

	if (scrollRef) {
	// parent's ref doesn't exist when useLayoutEffect is called
	microtask(() => assignScrollableElement(scrollRef[refKey]!));
	} else {
	assignScrollableElement(containerRef[refKey]!.parentElement!);
	}

I just tried to implement it for Vue in #393. However, it is not working correctly at the moment. I'm not an expert of Vue so maybe something is wrong. Maybe we have to implement it in a different way than React.