Ensure ImageResponse extends Response (#55187)

### Why?

The `ImageResponse` class constructor returns a `Response` instance, but doesn't extend `Response`, so it doesn't pass the type check introduced in #51394.

### How?

Make the `ImageResponse` class a subclass of `Response`.

### Notes

I can't find any tests to check for the types so didn't add any tests here; but shouldn't there be one?

packages/next/src/server/web/spec-extension/image-response.ts

@@ -1,4 +1,4 @@
-export class ImageResponse {
+export class ImageResponse extends Response {
   public static displayName = 'NextImageResponse'
   constructor(
     ...args: ConstructorParameters<
@@ -36,7 +36,7 @@ export class ImageResponse {
 
     const options = args[1] || {}
 
-    return new Response(readable, {
+    super(readable, {
       headers: {
         'content-type': 'image/png',
         'cache-control':