Add support (or disclose non-support) for `onopen` / `onmessage` / `onerror`

[morleyzhi]

The underlying polyfill supports onerror / onmessage / onopen:

https://github.com/jordanbyron/react-native-event-source/blob/master/EventSource.js#L202 https://github.com/jordanbyron/react-native-event-source/blob/master/EventSource.js#L179

But this library doesn't detect when onerror / onmessage / onopen are set:

https://github.com/jordanbyron/react-native-event-source/blob/master/index.js

[ghasemikasra39]

+1 need support

[rizkisunaryo]

I need help on this as well 🙏

[felansu]

+1 need that