Use IntersectionObserver when available

[pocketjoso]

Eventually we should be able to replace the window scroll listener with Intersection Observer API usage:
https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API

However note that it's not very likely that this would have any impact at all on performance, as the bottle neck for this component is the number of re-paints of the color background element that we perform.

Also note that the Intersection Observer API is currently not yet supported in safari, so would need to keep current scroll listener logic as fallback.