feat: add support for v3.x clients

In order to ease the migration to Socket.IO v3, the Engine.IO server
can now communicate with v3.x clients.

```js
const eioServer = require("engine.io")(httpServer, {
  allowEIO3: true // false by default
});
```

If `allowEIO3` is false, the v3.x clients will now receive an HTTP 400
response ("Unsupported protocol version").

Note: the code of the v3 parser has been imported from [1] and
browser-related dependencies were removed.

[1]: https://github.com/socketio/engine.io-parser/tree/2.2.1

Related:

- socketio/engine.io-protocol#35
- socketio/socket.io-protocol#21

.prettierignore

@@ -0,0 +1 @@
+lib/parser-v3/

README.md

@@ -252,6 +252,7 @@ to a single process.
       - `wsEngine` (`String`): what WebSocket server implementation to use. Specified module must conform to the `ws` interface (see [ws module api docs](https://github.com/websockets/ws/blob/master/doc/ws.md)). Default value is `ws`. An alternative c++ addon is also available by installing `uws` module.
       - `cors` (`Object`): the options that will be forwarded to the cors module. See [there](https://github.com/expressjs/cors#configuration-options) for all available options. Defaults to no CORS allowed.
       - `initialPacket` (`Object`): an optional packet which will be concatenated to the handshake packet emitted by Engine.IO.
+      - `allowEIO3` (`Boolean`): whether to support v3 Engine.IO clients (defaults to `false`)
 - `close`
     - Closes all clients
     - **Returns** `Server` for chaining