Indicate that promise messages may be functions, fixes #106

[BenJenkinson]

• Define the types for the toast.promise options to better indicate that they can accept a function in the same way that toast.success and the other methods can.
• Add example to the documentation since a custom dismiss button seems to be a popular request.

Example

This already works in plain JS, but now it also works in TypeScript.

toast.promise(myPromise, {
  loading: "Loading",
  success: (data) => (toast) =>
    (
      <div>
        <span>Successfully saved ${data.name}</span>
        <button type="button" onClick={() => toast.dismiss(t.id)}>
          ⭐ Custom dismiss button in a promise toast
        </button>
      </div>
    ),
  error: (err) => `This just happened: ${err.toString()}`,
});

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