fix(types): Fix clone method type declaration to be an instance method rather than instance property (#4925)

Author: mistval

test/types/fetch.test-d.ts

@@ -188,3 +188,15 @@ expectNotAssignable<RequestInit>({ duplex: 'not valid' })
 expectType<string[]>(headers.getSetCookie())
 
 expectType<Request>(new Request('https://example.com', request))
+
+expectAssignable<Response>(new (class extends Response {
+  override clone () {
+    return this
+  }
+})())
+
+expectAssignable<Request>(new (class extends Request {
+  override clone () {
+    return this
+  }
+})('https://example.com'))

types/fetch.d.ts

@@ -193,7 +193,7 @@ export declare class Request extends BodyMixin {
   readonly signal: AbortSignal
   readonly duplex: RequestDuplex
 
-  readonly clone: () => Request
+  public clone (): Request
 }
 
 export interface ResponseInit {
@@ -223,7 +223,7 @@ export declare class Response extends BodyMixin {
   readonly url: string
   readonly redirected: boolean
 
-  readonly clone: () => Response
+  public clone (): Response
 
   static error (): Response
   static json (data: any, init?: ResponseInit): Response