Add typings for underlying field for engine.io

Currently, the underlying engine property typed [is typed as any](https://github.com/socketio/socket.io/blob/d3d0a2d5beaff51fd145f810bcaf6914213f8a06/lib/index.ts#L130), when it should be a better type, as [`engine.io`](https://github.com/socketio/engine.io) [is written in TypeScript](https://github.com/socketio/engine.io/search?l=typescript).
