shared: Describe interface of typing_status in Flow.

This allows the mobile app to stay well-typed while using this code.

static/shared/js/typing_status.js

@@ -30,8 +30,10 @@ var TYPING_STOPPED_WAIT_PERIOD = 5000; // 5s
     appropriately.)
 */
 
+/** Exported only for tests. */
 export const state = {};
 
+/** Exported only for tests. */
 export function initialize_state() {
     state.current_recipient =  undefined;
     state.next_send_start_time =  undefined;
@@ -40,6 +42,7 @@ export function initialize_state() {
 
 initialize_state();
 
+/** Exported only for tests. */
 export function stop_last_notification(worker) {
     if (state.idle_timer) {
         clearTimeout(state.idle_timer);
@@ -48,6 +51,7 @@ export function stop_last_notification(worker) {
     initialize_state();
 }
 
+/** Exported only for tests. */
 export function start_or_extend_idle_timer(worker) {
     function on_idle_timeout() {
         // We don't do any real error checking here, because
@@ -75,6 +79,7 @@ function actually_ping_server(worker, recipient, current_time) {
     set_next_start_time(current_time);
 }
 
+/** Exported only for tests. */
 export function maybe_ping_server(worker, recipient) {
     var current_time = worker.get_current_time();
     if (current_time > state.next_send_start_time) {

static/shared/js/typing_status.js.flow

@@ -0,0 +1,19 @@
+/**
+ * @flow strict
+ */
+
+"use strict";
+
+type RecipientUserIds = number[];
+
+type Worker = {|
+  get_recipient: () => RecipientUserIds | void,
+  is_valid_conversation: (RecipientUserIds | void) => boolean,
+  get_current_time: () => number, // as ms since epoch
+  notify_server_start: RecipientUserIds => void,
+  notify_server_stop: RecipientUserIds => void
+|};
+
+declare export function handle_text_input(Worker): void;
+
+declare export function stop(Worker): void;