Create app.d.ts file introduced with 1.0.0-next.257

Add App namespace for app-level types [#3670](sveltejs/kit#3670)

Author: indaco

Diff for: src/app.d.ts

@@ -0,0 +1,15 @@
+/// <reference types="@sveltejs/kit" />
+
+// See https://kit.svelte.dev/docs#typescript
+// for information about these interfaces
+declare namespace App {
+	interface Locals {
+		userid: string;
+	}
+
+	interface Platform {}
+
+	interface Session {}
+
+	interface Stuff {}
+}