StartStopNotifier type now requires update param

This will be a breaking change for anyone who uses the StartStopNotifier
type in their custom stores.

src/runtime/store/index.ts

@@ -13,7 +13,7 @@ export type Updater<T> = (value: T) => T;
 type Invalidator<T> = (value?: T) => void;
 
 /** Start and stop notification callbacks. */
-export type StartStopNotifier<T> = (set: Subscriber<T>, update?: (fn: Updater<T>) => void) => Unsubscriber | void;
+export type StartStopNotifier<T> = (set: Subscriber<T>, update: (fn: Updater<T>) => void) => Unsubscriber | void;
 
 /** Readable interface for subscribing. */
 export interface Readable<T> {