Provide way to access Watcher in onChange event#45

I would like to access the Watcher object from the onChange handler in the watch configuration input. This way, a user can create public methods on their FileWatchingBackend to use in onChange. ## Desired Behavior Not sure how to reference the watcher with `WatcherConstructable` being a constructor. Maybe pass a reference in ChangeEvent ``` onChange: async ({ spawn, files, first, abortSignal, watcher }: ChangeEvent) => { ``` ## Motivation I would like to conditionally add and remove paths from my chokidar watch. ``` public add(path: string) { this.chokidar.add(path); } public unwatch(path: string) { this.chokidar.unwatch(path); } ```