fix: allow mutation of data and form props in Svelte 5

By doing `bind:data/form`, the layout/child components are allowed to mutate those properties
Related sveltejs/svelte#9905

Author: dummdidumm

.changeset/selfish-seas-change.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/kit": patch
+---
+
+fix: allow mutation of data and form props in Svelte 5

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

@@ -21,16 +21,22 @@ export function write_root(manifest_data, output) {
 
 	let l = max_depth;
 
-	let pyramid = `<svelte:component this={constructors[${l}]} bind:this={components[${l}]} data={data_${l}} {form} />`;
+	let pyramid = `<svelte:component this={constructors[${l}]} bind:this={components[${l}]} ${
+		isSvelte5Plus() ? `bind:data={data_${l}} bind:form` : `data={data_${l}} {form}`
+	} />`;
 
 	while (l--) {
 		pyramid = dedent`
 			{#if constructors[${l + 1}]}
-				<svelte:component this={constructors[${l}]} bind:this={components[${l}]} data={data_${l}}>
+				<svelte:component this={constructors[${l}]} bind:this={components[${l}]} ${
+					isSvelte5Plus() ? `bind:data={data_${l}}` : `data={data_${l}}`
+				}>
 					${pyramid}
 				</svelte:component>
 			{:else}
-				<svelte:component this={constructors[${l}]} bind:this={components[${l}]} data={data_${l}} {form} />
+				<svelte:component this={constructors[${l}]} bind:this={components[${l}]} ${
+					isSvelte5Plus() ? `bind:data={data_${l}} bind:form` : `data={data_${l}} {form}`
+				} />
 			{/if}
 		`;
 	}