Support an anchor index

[rostero1]

Is your feature request related to a problem? Please describe.

Discord chat has a concept of an anchor. If a user scrolls up to a message it will set one of the messages in view to be the anchor. The result is that any contents above or below the anchor that have layout shift, such as someone deleting a message or an image loading on a slow connection, will not affect the scroll position of the anchored item. This could be helpful for keyboard focus too, but I haven't investigated into that.

Describe the solution you'd like

An anchorId prop.

Describe alternatives you've considered

I have not tried anything, but this maybe could be solved with my own resize observer, but I'm afraid I'm scrolling after virtua is scrolling. Or something is happening too late.