R session background connections

In #394, notebook.R runs a socket connection that the notebook communicates with. Would it be worth extracting part of this out so that a socket connection is available for each R session started by the user? This would make the following possible:

• Sending VS Code's R commands to the session invisibly, rather than having to send them as text to the console
• Querying the session, for example to get the names of data frames
• For Run Selection/Line, potentially using R instead of TypeScript to choose which lines to send (allowing use of R's parse functions)
• Inline evaluation (sending user code to the session invisibly, and displaying the results in the editor) - this one's not really necessary but some other languages/IDE's have it and it's kind of cool

@renkun-ken You wrote notebook.R and the session watcher, which has some overlap with this proposal, so you're definitely the best person to comment. Does this sound practical? Let me know if I need to clarify anything.

Start by reading notebook.R and the session watcher work referenced in #394, then review the proposed per-session socket use cases. The issue needs a decided scope and practical design before implementation can begin; done would require an agreed approach for session connections and its supported commands.