Add virtual rendering to timeline rows when scrolling

[camargo]

For timelines with potentially many rows, rendering rows that are out of view is an inefficient use of browser resources. This ticket is to implement a virtual rendering scheme when scrolling that only adds rows to the DOM that are currently in view for a timeline.

[camargo]

Here is an existing virtual list component we could look at using: https://github.com/sveltejs/svelte-virtual-list

[duranb]

Could also look into using something like: https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API