[core] Remove keycode()

[oliviertassinari]

All of keyCode, which, charCode and keyIdentifier are deprecated. The key property is the new standard.
As of the time of this writing, the key property is supported by all major browsers as of : Firefox 52, Chrome 55, Safari 10.1, Opera 46. Except Internet Explorer 11 which has : non-standard key identifiers.
React synthetic event normalize all of that: https://github.com/facebook/react/blob/10a7a5b5ced683125d68584a78084faac3197846/packages/react-dom/src/events/getEventKey.js#L79 so we can apply this simple change:

-import keycode from 'keycode';

-const key = keycode(event);
+const key = event.key;

🚀 Removing the keycode dependency saves us 860 B gzipped.