Feature request: Initial "start" value (auto-scroll to certain element)

[Kreozot]

I really need to auto-scroll the list to remembered position. But the algorythm is kinda complicated. So I failed to do it myself so far. Maybe somebody can give me a hint how to achieve this behaviour. Or even make it if it is not so complicated.

I did almost all needed changes (as I think :)), but part after "prevent jumping if we scrolled up into unknown territory" is ruin everything, since I don't know previous row heights.

Thank you!

Diff by my code so far: https://github.com/sveltejs/svelte-virtual-list/compare/master...Kreozot:initial-position?expand=1

[Kreozot]

Also I think, I made some mistake. Because scroll is jumping even if I scroll down after initial render.