feat(ts): add ts declaration

[Armanio]

I tried to declare all methods, but I can wrong. Feel free to point out errors.
Thank you!

[Birowsky]

Thanx for the effort, this is not published, right?

I took a look at the commit, however, the onMessageReceived payload is not typed.

Can you please let me know whether I inferred the correct type based on the docs?

type Notification //payload type
  = FirebaseNotifIos
  | FirebaseNotifAndroid

type FirebaseNotifIos = MessageType & {
  aps: { alert: { title: string, body: string } }
  payload: string
}

type FirebaseNotifAndroid = MessageType & {
  title?: string
  body?: string
  payload: string
}

type MessageType
  = { messageType: 'data' }
  | { messageType: 'notification', tap: 'background' | 'foreground' };