Add Event.relationsCreated event to listen for future relations collections

If you ask for relations but none currently exist, we return `null` to avoid the
overhead of many empty relations objects in memory. However, we still want some
way to alert consumers when one _is_ later made, so this event level event
provides that.

Part of element-hq/element-web#9572
Part of element-hq/element-web#9485

Author: jryans

src/models/event-timeline-set.js

@@ -746,6 +746,10 @@ EventTimelineSet.prototype._aggregateRelations = function(event) {
             eventType,
             this.room,
         );
+        const relatesToEvent = this.findEventById(relatesToEventId);
+        if (relatesToEvent) {
+            relatesToEvent.emit("Event.relationsCreated", relationType, eventType);
+        }
     }
 
     relationsWithEventType.addEvent(event);