Start R language server only when needed and stop after 30s of inactivity

- Start the language server only when an R file is opened.
- Stop the server automatically 30 seconds after the last R file is closed.
- Cancel the shutdown timer if a new R file is opened within those 30 seconds.
- Fix "server disconnected" errors by double-checking that no R files are open exactly when the 30-second timer finishes, preventing the server from being accidentally killed while in use.

Author: Fred-Wu