feat: allow more types of overloads of mockResponse (#26)

This PR makes it possible to use more types of overloaded functions:

```ts
fetch.mockResponseOnce('x');
fetch.mockResponseOnce({ body: 'x' }); // previously not allowed
fetch.mockResponseOnce(new Response('x'));  // previously not allowed

fetch.mockResponseOnce(() => 'x');
fetch.mockResponseOnce(() => ({ body: 'x' }));
fetch.mockResponseOnce(() => new Response('x'));

fetch.mockResponseOnce(() => Promise.resolve('x'));
fetch.mockResponseOnce(() => Promise.resolve({ body: 'x' }));
fetch.mockResponseOnce(() => Promise.resolve(new Response('x')));
```

This could make certain cases less awkward, like
#25 (comment):
```ts
fetch.mockResponseOnce({ status: 204 });
fetch.mockResponseOnce(new Response(null, { status: 204 }));
```