fix: type imports in shielding.d.ts

Replaced top-level import with inline import() to properly
reference Backend type

Author: cromery

types/shielding.d.ts

@@ -1,10 +1,10 @@
-import { Backend } from 'fastly:backend';
+/// <reference path="../types/backend.d.ts" />
 
 declare module 'fastly:shielding' {
   export class Shield {
     constructor(name: string);
     runningOn(): boolean;
-    unencryptedBackend(): Backend;
-    encryptedBackend(): Backend;
+    unencryptedBackend(): import('fastly:backend').Backend;
+    encryptedBackend(): import('fastly:backend').Backend;
   }
 }