Virtualizer should support scrolling detection for items

[ling1726]

Items should be able to know that scrolling is happening so that they can optionally render a more 'lightweight' variant to increase scrolling performance and decrease whitespace.

A similar feature exists in react-window

[brandonthomas]

@Mitch-At-Work since there's Suspense support is this handled?

[Mitch-At-Work]

Can close thanks to #27864 - user can set a flagged index in any virtualizer and request access to the virtualizer index/length to detect current range, as well as be notified when the scrollview reaches it's scrolledTo call via callback.