Typings for createEventDispatcher

Makes it possible to explicitly type which events can be dispatched like so:

```ts
const bla2 = createEventDispatcher<{click: boolean}>();
bla2('click', ''); // error, type string not assignable to type boolean
bla2('qwd', true); // error, "qwd" not assignable to "click"
```

#5211

src/runtime/internal/lifecycle.ts

@@ -27,7 +27,9 @@ export function onDestroy(fn) {
 	get_current_component().$$.on_destroy.push(fn);
 }
 
-export function createEventDispatcher() {
+export function createEventDispatcher<
+	EventMap extends {} = any
+>(): <EventKey extends Extract<keyof EventMap, string>>(type: EventKey, detail?: EventMap[EventKey]) => void {
 	const component = get_current_component();
 
 	return (type: string, detail?: any) => {
@@ -61,4 +63,4 @@ export function bubble(component, event) {
 	if (callbacks) {
 		callbacks.slice().forEach(fn => fn(event));
 	}
-}
+}