added fetch api type definitions extracted from official dom lib usin…

…g ts-graft (#93)

lib.fetch.d.ts includes all types from the dom lib needed to define the
fetch function and the Request, Response, and Headers classes. The file
was generated by ts-graft as follows:

npm i --no-save typescript@4.2.3 \
&& cat <<EOF > .ts-graftrc.yaml \
&& npx ts-graft@1.0.1 \
&& rm .ts-graftrc.yaml \
&& npm un typescript
grafts:
- source: typescript/lib/lib.dom.d.ts
  output: lib.fetch.d.ts
  include: [RequestInfo, RequestInit, Response]
EOF

index.d.ts

@@ -1,14 +1,27 @@
-/// <reference lib="dom" />
+import {
+  Headers,
+  Request,
+  RequestInfo,
+  RequestInit,
+  Response,
+} from "./lib.fetch"
 
-declare const _fetch: typeof fetch;
-declare const _Request: typeof Request;
-declare const _Response: typeof Response;
-declare const _Headers: typeof Headers;
 
 declare module "cross-fetch" {
-  export const fetch: typeof _fetch;
-  export const Request: typeof _Request;
-  export const Response: typeof _Response;
-  export const Headers: typeof _Headers;
+  export const fetch: (input: RequestInfo, init?: RequestInit) => Promise<Response>;
+  export const Request: {
+    prototype: Request;
+    new(input: RequestInfo, init?: RequestInit): Request;
+  };
+  export const Response: {
+    prototype: Response;
+    new(body?: BodyInit | null, init?: ResponseInit): Response;
+    error(): Response;
+    redirect(url: string, status?: number): Response;
+  };
+  export const Headers: {
+    prototype: Headers;
+    new(init?: HeadersInit): Headers;
+  };
   export default fetch;
 }