add tests asserting on types of that callback function receives

[chodorowicz]

add a test that the asserts on existing behavior of callback functions

[urbanfly]

Now the declared type of the return value will be () => Promise<Promise<number>> which is not really a valid type.

Extend it a bit:

const fn = debounce(() => Promise.resolve(42), 100, {
  callback: (data: Promise<number>) => {
    expectType<Promise<number>>(data);
  },
});

const result = await fn();
// result === 42
// the function type claims that it will be a Promise<number>, which is impossible

[urbanfly]

I now realize that you understand the point I was making above - and I didn't get your point about the callback option. This PR, combined with #68 should be a good combo.