Improve WebSocket API

[nhooyr]

The net.Conn interface doesn't match how WebSockets work as it's not a stream oriented protocol.

See golang/go#18152 (comment) and gopherjs/websocket#29

I think it'd be a good idea to recommend my library instead to avoid duplicate work. It supports compiling to Wasm and provides a more semantically appropriate API (while also providing a net.Conn adapter) or the API in this library should be changed.