[minor] Add WebSocket.WebSocketServer alias

Add `WebSocket.WebSocketServer` as an alias for `WebSocket.Server` to
fix name consistency and improve interoperability with the ES module
wrapper.

Refs: #1932

index.js

@@ -7,4 +7,6 @@ WebSocket.Server = require('./lib/websocket-server');
 WebSocket.Receiver = require('./lib/receiver');
 WebSocket.Sender = require('./lib/sender');
 
+WebSocket.WebSocketServer = WebSocket.Server;
+
 module.exports = WebSocket;