feat(ios): addDelegateHandler to add App Delegate handlers

[rigor789]

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What is the current behavior?

In order to add delegate handlers, users must replace the Application.ios.delegate with their own - accounting for existing delegates if any (ie. from plugins).

What is the new behavior?

To simplify adding delegate handlers, users no longer need to replace the delegate, and can just add/register them via the new api:

Application.ios.addDelegateHandler('applicationDidBecomeActive', (application: UIApplication) => {
  console.log('applicationDidBecomeActive', application);
});

The types should automatically be inferred for the handler function.

Implementation loosely based off of https://github.com/nativescript-community/universal-links/blob/master/src/universal-links/index.ios.ts#L6-L31