Add "Load more" button to the top of timeline instead of having to wrestle the scroll back and fourth to trigger infinite scroll

### Your use case

#### What would you like to do?

Add a "Load more" button to the top of timeline instead of having to wrestle the scroll back and fourth to trigger infinite scroll. Basically an alternative to the intersection observer. Inspired by https://gitlab.com/gitterHQ/webapp/-/issues/2289


#### Why would you like to do it?

When I want to scroll back many days, I keep scrolling up over and over and over. Oftentimes, the infinite loading does not trigger unless I scroll back down and up to hit the top. In any case of the infinite scrolling problem being fixed, it would be nice to add a manual action to go and fetch the next page of scrollback.

This helps when the timeline fails in any case and for any of the future regressions and bugs:

- https://github.com/vector-im/element-web/issues/18325
- https://github.com/vector-im/element-web/issues/23208

### Have you considered any alternatives?

_No response_

### Additional context

UI References

Google photos has infinite scrolling and "Show more results" button once you hit the soft-limit: