Timeline: add a Jump to Unread button and a new-message badge on the scroll-to-bottom button. (#5506)

* Timeline: add a Jump to Unread button and a new-message badge on the scroll-to-bottom button.

* Rename scrollToFirstUnread to scrollToReadMarker for consistency with isReadMarkerVisible.

* Cache readMarkerUniqueID and unreadMessageCount as stored properties on TimelineState.

* TimelineViewModel: rename newTimelineItems and drop redundant ordering comment.

* Use Compound chevron icons in TimelineScrollButton.

* Fix TimelineScrollButton layout at large dynamic type sizes.

* Add a trailingAccessory slot to topBanners and use it to dodge banners with the jump-to-read-marker button.

* Put jump-to-read-marker behind a developer-options feature flag.

* Fine-tune TimelineScrollButton sizing and badge colour.

* Remove TimelineScrollButton chevron offset that was compensating for SF Symbol centering.

* Drop the new-messages count, replace with a presence dot.

* Move both timeline scroll buttons to bottom-right and revert topBanners trailingAccessory slot.

* Position TimelineScrollButton badge above for up direction and below for down direction.

* Add long-press mark-as-read pill to the timeline scroll buttons

* Read isSwitchingTimelines from state in updateHasNewMessagesAtBottom.

* Use assertionFailure when scrollToReadMarker is called with no read marker.

* Add TimelineScrollButton preview snapshots.

* (fix) design and various self-review issues

* (fix) suppress scroll button tap after long-press

* (feat) fall back to RoomInfo.fullyReadEventID for jump-to-unread

* (fix) hide jump-to-unread immediately after mark-as-read

* (fix) align with newer matrix-rust-sdk surface

* Bump the RustSDK to v26.05.20

---------

Signed-off-by: Daniel Anderson <daniel.anderson@toptal.com>
Co-authored-by: Stefan Ceriu <stefan.ceriu@gmail.com>

ElementX.xcodeproj/project.pbxproj

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