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.

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

libraries/featureflag/impl/src/main/kotlin/io/element/android/libraries/featureflag/impl/StaticFeatureFlagProvider.kt

@@ -38,6 +38,7 @@ class StaticFeatureFlagProvider @Inject constructor() :
                 FeatureFlags.VoiceMessages -> false
                 FeatureFlags.PinUnlock -> false
                 FeatureFlags.InRoomCalls -> false
+                FeatureFlags.Mentions -> false
             }
         } else {
             false