fix: lib.fetch types using experimental ts-graft discover mode

[jstewmon]

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 7b19cdab1c9b84294032f16a2b13ec258f3a726e -- 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.

[lquixada]

thanks @jstewmon ! I feel there's still some work to do here such as adding a job on CI to prevent further type issues but this PR is a good step!