Function extract_event_relation 

pub fn extract_event_relation(
    event: &Raw<AnySyncTimelineEvent>,
) -> Option<(OwnedEventId, String)>

Helper to extract the relation information from an event.

If the event isn’t in relation to another event, then this will return None. Otherwise, returns both the event id this event relates to, and the kind of relation as a string (e.g. m.replace).