chore: fix preview site (#10519)

Co-authored-by: Rich Harris <rich.harris@vercel.com>

Author: Rich-Harris

sites/svelte-5-preview/src/lib/Output/Viewer.svelte

@@ -131,9 +131,9 @@
 
 					const __repl_exports = ${$bundle.client?.code};
 					{
-						const { mount, App } = __repl_exports;
-						const [, destroy] = mount(App, { target: document.body });
-						window.__unmount_previous = destroy;
+						const { mount, unmount, App } = __repl_exports;
+						const component = mount(App, { target: document.body });
+						window.__unmount_previous = () => unmount(component);
 					}
 					//# sourceURL=playground:output
 				`);

sites/svelte-5-preview/src/lib/workers/bundler/index.js

@@ -485,7 +485,7 @@ async function bundle({ uid, files }) {
 	lookup.set('./__entry.js', {
 		name: '__entry',
 		source: `
-			export { mount } from 'svelte';
+			export { mount, unmount } from 'svelte';
 			export {default as App} from './App.svelte';
 		`,
 		type: 'js',