ProtocolClient.send breaks async stacktraces

[int19h]

The following implementation: https://github.com/microsoft/vscode-debugadapter-node/blob/1621133e71527d9d46438edb766eaf8e731b68f5/testSupport/src/protocolClient.ts#L69-L80 is not async itself and returns the promise immediately without awaiting it. Consequently, if the promise gets rejected with an error, the stack trace in the error does not include either send or its caller. This unnecessarily complicates debugging of e.g. async unit tests that await send() and receive an unexpected error response.

Instead, the callback for doSend should capture the raw response in a wrapper promise, and then send should await that promise and translate it to a throw.

[connor4312]

Makes sense, PR welcome if you're up for it