WebTransportBidirectionalStream：writable 属性
Limited availability
This feature is not Baseline because it does not work in some of the most widely-used browsers.
备注： 此特性在 Web Worker 中可用。
WebTransportBidirectionalStream 接口的只读属性 writable 返回一个可用于写入传出数据的 WebTransportSendStream 实例。