improve WASM support

[marten-seemann]

quic-go works when given a net.PacketConn created using the WASI socket extension (https://github.com/dispatchrun/net). This is great news.

What doesn't work yet is using the convenience functions (e.g. DialAddr and ListenAddr), as these functions call into the Go standard library net package (which doesn't support WASM). We could add a build flag and call into github.com/dispatchrun/net instead.

However, this will create a dependency of quic-go on that library, so maybe we don't want to do that?