Add ability for making server->client requests to `Server.RequestM`

[Vtec234]

Currently Server.RequestM does not provide any way to make a server->client LSP request and then wait on the client's response. This is useful in cases where we want to make one request, wait until it resolves, and then make another which relies on the client state being updated by the first request.

The implementation does not seem too easy. It would probably require plumbing in the worker's pending requests table, maybe storing an extra FS.Stream or something to write the response into so that the Task can receive it, as well as handling responses in the worker and in the watchdog.