readableStream.tee()
返回一对新的 <ReadableStream> 实例，该 ReadableStream 的数据将被转发到这两个实例。每个实例都会接收到相同的数据。
【Returns a pair of new <ReadableStream> instances to which this
ReadableStream's data will be forwarded. Each will receive the
same data.】
导致 readableStream.locked 为 true。
【Causes the readableStream.locked to be true.】