fix: set runes option in generated root (#11111)

Author: dummdidumm

.changeset/fast-berries-shake.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: set runes option in generated root

packages/kit/src/core/sync/write_root.js

@@ -39,6 +39,7 @@ export function write_root(manifest_data, output) {
 		`${output}/root.svelte`,
 		dedent`
 			<!-- This file is generated by @sveltejs/kit — do not edit it! -->
+			${isSvelte5Plus() ? '<svelte:options runes={true} />' : ''}
 			<script>
 				import { setContext, ${isSvelte5Plus() ? '' : 'afterUpdate, '}onMount, tick } from 'svelte';
 				import { browser } from '$app/environment';