add NotificationEvent type
add NotificationEvent type

Summary:
Add NotificationEvent type definition
Spec: https://notifications.spec.whatwg.org/#notificationevent

Reviewed By: mroch

Differential Revision: D35530404

fbshipit-source-id: 8b8157f48b3690fa899d752771aea11cb53f6291

facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow