[fix] call onDestroy in SSR

Via #6416, lifecycle hooks became noops for SvelteKit. But onDestroy is said to be called suring SSR according to the docs.

Fixes #6676

src/runtime/ssr.ts

@@ -1,4 +1,5 @@
 export {
+	onDestroy,
 	setContext,
 	getContext,
 	getAllContexts,
@@ -10,6 +11,5 @@ export {
 } from './index';
 
 export function onMount() {}
-export function onDestroy() {}
 export function beforeUpdate() {}
 export function afterUpdate() {}