We could make session watcher script initialization more robust with the following applied:

• init.R detects if environment variable TERM_PROGRAM=vscode. Only in VSCode it is effective.
• ~/.Rprofile only sources init.R when ~/.vscode-R/init.R exists. Disabling r.sessionWatcher removes ~/.vscode-R/init.R so that user does not have to comment out or remove the source() line manually.

The only drawback of detecting TERM_PROGRAM=vscode is that if R is started in a pre-existing environment inside a terminal multiplexer (e.g. tmux and screen) whose server program is not started inside VSCode terminal, it won't get a TERM_PROGRAM=vscode environment variable. But user could always set this environment variable to force it.

This is related to #190, #195, randy3k/radian#139. When user tries to disable session watcher by setting "r.sessionWatcher": false but leaves source(...) line in ~/.Rprofile or any other startup script, the use of all R sessions will be broken.