fix: Update `$derived.call` type declaration (#10349)

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/honest-dragons-turn.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+Use generic `T` as the return type for `$derived.call()`

packages/svelte/src/main/ambient.d.ts

@@ -77,7 +77,7 @@ declare namespace $derived {
 	 *
 	 * https://svelte-5-preview.vercel.app/docs/runes#$derived-call
 	 */
-	export function call<T>(fn: () => T): void;
+	export function call<T>(fn: () => T): T;
 }
 
 /**

packages/svelte/types/index.d.ts

@@ -2511,7 +2511,7 @@ declare namespace $derived {
 	 *
 	 * https://svelte-5-preview.vercel.app/docs/runes#$derived-call
 	 */
-	export function call<T>(fn: () => T): void;
+	export function call<T>(fn: () => T): T;
 }
 
 /**