Add support for WebSockets on Qt for WASM

Implement the QWebSocket API using HTML WebSocket.

Change-Id: I2284f8606d512943d2abc2a96fc48effcc45a786
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>