Running various applications that use jsonrpc2 with the Go race detector shows that there is a race condition where `WriteObject` can be called from concurrent goroutines (e.g., 1 sending a request, 1 writing a response).
simplify API by using interface{}, rename from transport -> stream
add WebSocket transport in websocket subpackage
do not buffer in ReadObject, rename GetObjectReader/ReadObject -> NextObjectReader
use xtest (jsonrpc2_test) package to allow us to test subpackages that depend on us (in a future change)
factor out vscode-specific transport code and allow pluggable transports
remove Server (unused) and Serve (unnecessary):
The Serve func had nothing specific to JSON-RPC; it was just a loop
around (net.Listener).Accept. It added no value.
