Initial support for member suggestions (#1631)

* Initial support for member suggestion (search and UI)

* Add custom `BottomSheetScaffold` implementation to workaround several scrolling bugs

* Start searching as soon as `@` is typed, add UI following initial designs

* Extract suggestion processing code

* Extract component, add previews, fix tests

* Add tests

* Add exception from kover to the forked bottom sheet code

* Add a feature flag for mentions

- Extract composer & mention suggestions to their composable.
- Extract mentions suggestions processing to its own class.
- Add `MatrixRoom.canTriggerRoomNotification` function.
- Update strings and conditions for displaying the `@room` mention.

---------

Co-authored-by: ElementBot <benoitm+elementbot@element.io>

libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlags.kt

@@ -60,5 +60,11 @@ enum class FeatureFlags(
         title = "Element call in rooms",
         description = "Allow user to start or join a call in a room",
         defaultValue = false,
+    ),
+    Mentions(
+        key = "feature.mentions",
+        title = "Mentions",
+        description = "Type `@` to get mention suggestions and insert them",
+        defaultValue = false,
     )
 }