virtual-scroll: allow setting the scrolled-to index

Add an input to the virtual scroll container that allows developers to set the index of the first element that should be visible in the viewport.

We should also allow specifying an index that is greater than the length of the currently loaded data. Once the data loads we can scroll to the specified index.

Start by locating the virtual scroll container entry point and how its current viewport index is managed. Trace the data-loading path to understand how a requested index beyond the loaded data could be retained. Done means developers can set the first visible index and the container scrolls to that index once the required data loads.

Written by the indexing model from the issue text.