mark expiration as required (#8943)

Author: Rich-Harris

.changeset/long-timers-try.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+fix: mark `expiration` as required

packages/adapter-vercel/index.d.ts

@@ -35,7 +35,7 @@ export interface ServerlessConfig {
 		/**
 		 * Expiration time (in seconds) before the cached asset will be re-generated by invoking the Serverless Function. Setting the value to `false` means it will never expire.
 		 */
-		expiration?: number | false;
+		expiration: number | false;
 		/**
 		 * Option group number of the asset. Assets with the same group number will all be re-validated at the same time.
 		 */