typing_status: Add an extra notification_event_type to Recipients.

This is a preparatory commit. It adds an extra notification_event_type
to the Recipient type. This can be used to differentiate between
typing notifications and message edit typing notifications since
they send different events.

web/shared/src/typing_status.ts

@@ -8,10 +8,12 @@ type StreamTopic = {
 export type Recipient =
     | {
           message_type: "direct";
+          notification_event_type: "typing";
           ids: number[];
       }
     | (StreamTopic & {
           message_type: "stream";
+          notification_event_type: "typing";
       });
 type TypingStatusWorker = {
     get_current_time: () => number;