add mw.now, mw.requestIdleCallback, mw.track, etc

Not covered in https://doc.wikimedia.org/mediawiki-core/master/js/, so the doc comments are copied from source code.

mw/index.d.ts

@@ -39,6 +39,98 @@ declare global {
 		 */
 		function format(formatString: string, ...parameters: unknown[]): string;
 
+		/**
+		 * Get the current time, measured in milliseconds since January 1, 1970 (UTC).
+		 *
+		 * On browsers that implement the Navigation Timing API, this function will produce
+		 * floating-point values with microsecond precision that are guaranteed to be monotonic.
+		 * On all other browsers, it will fall back to using `Date`.
+		 *
+		 * @return {number} Current time
+		 */
+		function now(): number;
+
+		/**
+		 * Schedule a deferred task to run in the background.
+		 *
+		 * This allows code to perform tasks in the main thread without impacting
+		 * time-critical operations such as animations and response to input events.
+		 *
+		 * Basic logic is as follows:
+		 *
+		 * - User input event should be acknowledged within 100ms per [RAIL].
+		 * - Idle work should be grouped in blocks of upto 50ms so that enough time
+		 *   remains for the event handler to execute and any rendering to take place.
+		 * - Whenever a native event happens (e.g. user input), the deadline for any
+		 *   running idle callback drops to 0.
+		 * - As long as the deadline is non-zero, other callbacks pending may be
+		 *   executed in the same idle period.
+		 *
+		 * See also:
+		 *
+		 * - <https://developer.mozilla.org/en-US/docs/Web/API/Window/requestIdleCallback>
+		 * - <https://w3c.github.io/requestidlecallback/>
+		 * - <https://developers.google.com/web/updates/2015/08/using-requestidlecallback>
+		 * [RAIL]: https://developers.google.com/web/fundamentals/performance/rail
+		 *
+		 * @member mw
+		 * @param {Function} callback
+		 * @param {Object} [options]
+		 * @param {number} [options.timeout] If set, the callback will be scheduled for
+		 *  immediate execution after this amount of time (in milliseconds) if it didn't run
+		 *  by that time.
+		 */
+		function requestIdleCallback(callback: ((...args: any[]) => any)): void;
+
+		/**
+		 * Track an analytic event.
+		 *
+		 * This method provides a generic means for MediaWiki JavaScript code to capture state
+		 * information for analysis. Each logged event specifies a string topic name that describes
+		 * the kind of event that it is. Topic names consist of dot-separated path components,
+		 * arranged from most general to most specific. Each path component should have a clear and
+		 * well-defined purpose.
+		 *
+		 * Data handlers are registered via `mw.trackSubscribe`, and receive the full set of
+		 * events that match their subscription, including those that fired before the handler was
+		 * bound.
+		 *
+		 * @param {string} topic Topic name
+		 * @param {Object|number|string} [data] Data describing the event.
+		 */
+		function track(topic: string, data?: object | number | string): void;
+
+		/**
+		 * Track an early error event via mw.track and send it to the window console.
+		 *
+		 * @private
+		 * @param {string} topic Topic name
+		 * @param {Object} data Data describing the event, encoded as an object; see mw#logError
+		 */
+		function trackError(topic: string, data: object): void;
+
+		/**
+		 * Register a handler for subset of analytic events, specified by topic.
+		 *
+		 * Handlers will be called once for each tracked event, including any events that fired before the
+		 * handler was registered; 'this' is set to a plain object with a topic' property naming the event, and a
+		 * 'data' property which is an object of event-specific data. The event topic and event data are
+		 * also passed to the callback as the first and second arguments, respectively.
+		 *
+		 * @param {string} topic Handle events whose name starts with this string prefix
+		 * @param {Function} callback Handler to call for each matching tracked event
+		 * @param {string} callback.topic
+		 * @param {Object} [callback.data]
+		 */
+		function trackSubscribe(topic: string, callback: ((topic: string, data: object) => any)): void;
+
+		/**
+		 * Stop handling events for a particular handler
+		 *
+		 * @param {Function} callback
+		 */
+		function trackUnsubscribe(callback: Function): void;
+
 		// types for mw.widgets are out of scope!
 		const widgets: any;
     }