fix: form action submit function callback return type

Allow for form action submit function callbacks to be asynchronous
functions.

packages/kit/types/index.d.ts

@@ -1466,7 +1466,7 @@ declare module '@sveltejs/kit' {
 				 * @param invalidateAll Set `invalidateAll: false` if you don't want the action to call `invalidateAll` after submission.
 				 */
 				update: (options?: { reset?: boolean; invalidateAll?: boolean }) => Promise<void>;
-		  }) => void)
+		  }) => MaybePromise<void>)
 	>;
 
 	/**