Add transport: server-sent events

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You want to:

• report a bug
• request a feature

Current behaviour

The transports available are http polling and websocket.

Steps to reproduce (if the current behaviour is a bug)

N/A

Expected behaviour

To have the option of a new transport available: sse (server-sent events)

Setup

• OS: Any
• browser: Any (Edge requires a polyfill)
• engine.io version: 3.4.0

Other information (e.g. stacktraces, related issues, suggestions how to fix)

Server-sent events have existed for many years now as an option to push events from the server to the browser [1]. It is the ideal mechanism for some use-cases and has the potential to greatly save on power usage on mobile devices [2]. This is the sole transport used by Mercure [3], a new protocol for real-time communications that I believe might become a direct competitor to Socket.io.

[1] https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events
[2] https://www.smashingmagazine.com/2018/02/sse-websockets-data-flow-http2/
[3] https://github.com/dunglas/mercure

No files or tests are named. Start by reading the linked MDN server-sent events documentation and comparing it with the existing HTTP polling and WebSocket transports. Done means SSE is available as a selectable transport and works across the stated browser environments, including Edge with its polyfill.