fix: lib.fetch types using experimental ts-graft discover mode (#100)

lib.fetch.d.ts includes all type definitions from the dom and
dom.iterable libs which were reachable from the original cross-fetch
index.d.ts file:

```
git checkout 7b19cda -- index.d.ts
```

lib.fetch.d.ts was generated with the following command:

```
cat <<EOF > .ts-graftrc.yaml \
&& npx ts-graft@2.0.0-1 \
&& rm .ts-graftrc.yaml \
grafts:
  - source: index.d.ts
    output: lib.fetch.d.ts
    include:
      - dom
      - dom.iterable
EOF
```

index.d.ts was then updated to its new state and verified with the
following command:

```
npx tsc --lib ES6 index.d.ts
```

Issue #95 was validated as follows:

```
cat <<EOF > test.ts \
&& npx tsc --target ES6 --moduleResolution node --noEmit  test.ts \
&& rm test.ts
import fetch from "./";
export const customFetch = (
  input: RequestInfo,
  init: RequestInit,
): Promise<Response> => {
  let url = "";
  url += input;
  return fetch(url, init);
};
EOF
```

n.b. consumers must include the dom.iterable lib, which is implied by
the ES6 target, lest their globals lack the required members defined
by dom.iterable.

index.d.ts

@@ -1,33 +1,7 @@
-import type {
-  BodyInit,
-  Headers as BaseHeaders,
-  HeadersInit,
-  Request,
-  RequestInfo,
-  RequestInit,
-  Response,
-  ResponseInit,
-} from "./lib.fetch"
-import type { Headers as IterHeaders } from "./lib.fetch.iterable";
-type Headers = BaseHeaders & IterHeaders;
-
-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;
-};
+import * as libFetch from "./lib.fetch";
 
+export const fetch: typeof libFetch.fetch;
+export const Request: typeof libFetch.Request;
+export const Response: typeof libFetch.Response;
+export const Headers: typeof libFetch.Headers;
 export default fetch;