DEPRECATED: Start a socket and listen on it.

This namespace is deprecated and is scheduled for deletion. See the README for
recommendations on how to communicate with an LSP client over a socket.

This namespace was implemented based on a misconception. The LSP spec says
servers should accept a `--port` CLI argument for socket-based communication.
We assumed that the server was expected to start a socket server on that port,
then wait for the client to connect and start sending messages. However, this
is not how clients actually work. Instead, the _client_ opens the socket
server, and tells the LSP server which port to connect to. That is, the LSP
client acts as a socket server and the LSP server acts as a socket client.

This namespace, which largely deals with starting a socket server, is
therefore unusable in an LSP server. If the LSP server did try to open a
server on the provided `--port`, it should always fail, because the client
should already be listening on that port.