Changes contract of void functions

[turadg]

This is most evident in React lifecycle methods like componentDidMount which is transforms to an async method. (Currently componentDidMount async function due to #3 )

Regardless of whether it's for React, doesn't seem prudent for this codemod to change the contract of the code. E.g.,

function doesSomethingWithAPromise(): void {
  promise.then(doSomething);
}

is being converted to:

async function doesSomethingWithAPromise(): Promise<void> {
  await promise;
  doSomething();
}

(hypothetically, haven't tested with this particular snippet) Point is that the return type has been changed from void to Promise<void>.