Executes async script on page.
Provided function should execute a passed callback (as first argument) to signal it is finished.

Example: In Vue.js to make components completely rendered we are waiting for [nextTick](https://vuejs.org/v2/api/#Vue-nextTick).

```js
I.executeAsyncScript(function(done) {
  Vue.nextTick(done); // waiting for next tick
});
```

By passing value to `done()` function you can return values.
Additional arguments can be passed as well, while `done` function is always last parameter in arguments list.

```js
let val = await I.executeAsyncScript(function(url, done) {
  // in browser context
  $.ajax(url, { success: (data) => done(data); }
}, 'http://ajax.callback.url/');
```

@param {string|function} fn function to be executed in browser context.
@param {...any} args to be passed to function.
@return {Promise<any>}