Add ID_RESPONSE Message, to let users know their authorID

This value should be provided when sending messages, but will not be
trusted by the server.

ServerMessage.ts

@@ -1,6 +1,7 @@
 export enum MessageType {
   ACK = 0,
   TEXT = 1,
+  ID_RESPONSE = 3,
 }
 
 export type ServerMessage = {
@@ -71,3 +72,18 @@ export type AckMessage = ServerMessage & {
 export function isAckMessage(obj: unknown): obj is AckMessage {
   return isServerMessage(obj, MessageType.ACK);
 }
+
+export type IdResponseMessage = ServerMessage & {
+  type: MessageType.ID_RESPONSE;
+  authorId: string;
+};
+export function isIdResponseMessage(obj: unknown): obj is IdResponseMessage {
+  if (!isServerMessage(obj, MessageType.ID_RESPONSE)) return false;
+  if (
+    !Object.hasOwnProperty.call(obj, "authorId") ||
+    typeof (obj as ServerMessage & { authorId: unknown }).authorId !== "string"
+  ) {
+    return false;
+  }
+  return true;
+}