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.xcworkspace/xcshareddata/swiftpm/Package.resolved

@@ -1,5 +1,5 @@
 {
-  "originHash" : "29630e4198750ff70c84dd9ae8d634528fa2551a48536caca7c538a2fe40a243",
+  "originHash" : "03817e609ee3e9e372d9320f569c830d82dd6dd0f4455b29822903349a85c61e",
   "pins" : [
     {
       "identity" : "compound-design-tokens",
@@ -167,8 +167,8 @@
       "kind" : "remoteSourceControl",
       "location" : "https://github.com/element-hq/matrix-rust-components-swift",
       "state" : {
-        "revision" : "0e7dd117d013f2e2305552b68c7ad925c4421d0e",
-        "version" : "26.5.18"
+        "revision" : "5dd7f23990c9568f577152a932ed03111d8d2551",
+        "version" : "26.5.20"
       }
     },
     {