Improve specificity of return type

[nickiaconis]

Increases the specificity of the return type. This allows consumers to make assumptions about the existence of value based on the value/presence of loading and error.

For example, this will produce a type error with the current version of react-promise, but will work as expected with these changes:

function processMessage(input: string): string;

const messagePromise: Promise<string> = doXHRGetString();
const asyncMessage = usePromise(messagePromise);
const displayValue = asyncMessage.loading
  ? 'loading'
  : asyncMessage.error
  ? asyncMessage.error.toString()
  : processMessage(asyncMessage.value); // type error here with current version b/c `value` is type `string | undefined`

[nickiaconis]

Alternatively, this could throw a type error instead b/c a falsy value in promiseOrFn doesn't satisfy the type specified for the hook argument. Throwing an error might be better than setting an error on the state since this has the potential to be incorrectly interpreted as a rejected promise. 🤷 Would like your input on this please.