New timeline (#276) (#280)

* Fixes #276 - Rebuilt room timeline:
    - Removed the need for the ListCollectionViewAdapter
    - Rewrote the TimelineItemList without using introspection
    - Added ReversedScrollView for laying out items at the bottom/trailing
    - Rewrote TimelineProvider diffing through CollectionDifference (similar to the RoomSummaryProvider)
    - Added back `scrollDismissesKeyboard`  behavior
    - Various other tweaks and fixes
- Fixed various warnings:
    - removed async AttributedStringBuilder as AttributedString is non-sendable, made the RoomTimelineItemFactory synchronous
    - removed unused virtual timeline items
    - removed unused isOutgoing property from the FormattedBodyText
* Make TimelineItemContextMenuActions indentifiable and specify contextMenu identifiers
* Bump the matrix-rust-components-swift to v1.0.16-alpha
* Add changes file and changelog contribution guide
* Fix attributed string builder unit tests

ElementX.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved

@@ -86,8 +86,8 @@
       "kind" : "remoteSourceControl",
       "location" : "https://github.com/matrix-org/matrix-rust-components-swift",
       "state" : {
-        "revision" : "e3ae8941c864ba18e7e5c51d25a6ce2c5c7a65a0",
-        "version" : "1.0.15-alpha"
+        "revision" : "af6683ca5ddd9da7f582de4258e4d6ebb8a8caff",
+        "version" : "1.0.16-alpha"
       }
     },
     {