Support ability to add events to both top and bottom of TimelinePanel

[Palid]

Currently if we try to add an event to the timeline, the scroll position properly updates to include the bottom event, as TimelinePanel's scroll position expect that the only way you can add new events is to the end of the current timeline, which will always be the bottom of the view.
In case of ThreadPanel we are still adding events to the end of the Timeline, but the rendering order is reversed in comparison to the Room Timeline.

We need to properly account for the reversed rendering, so that the scroll position won't jump around when new events are added to the top of the view, instead of the bottom of it.

Related #18957
Follow-up to: matrix-org/matrix-react-sdk#6904