Fix: make shielding.d.ts ambient (#1256)

Author: cromery

test-d/shielding.test-d.ts

@@ -0,0 +1,9 @@
+/// <reference path="../types/shielding.d.ts" />
+import { Shield } from 'fastly:shielding';
+import { expectType, expectError } from 'tsd';
+
+// Shielding
+{
+  expectError(Shield());
+  expectType<Shield>(new Shield('dub-dublin-ie'));
+}

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;
   }
 }