[Rich text editor] Add feature flag for rich text editor (#1289)

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

@@ -44,4 +44,9 @@ enum class FeatureFlags(
         // Do not forget to edit StaticFeatureFlagProvider when enabling the feature.
         defaultValue = false,
     ),
+    RichTextEditor(
+        key = "feature.richtexteditor",
+        title = "Enable rich text editor",
+        defaultValue = true,
+    ),
 }