More robust session watcher initialization

[renkun-ken]

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.

[randy3k]

See also my comment #184 (comment)

[renkun-ken]

Thanks @randy3k!

Alternatively, one could make use of the env variable R_PROFILE_USER that points to a vscode generated .Rprofile. For example, vscode-R could generate ~/.vscode-R/.Rprofile and set R_PROFILE_USER to it in the integrated terminal.

Looks like we could use https://code.visualstudio.com/api/references/vscode-api#TerminalOptions to createTerminal so that the environment variable could be setup before the R terminal is started in VSCode terminal.

[stevenlis]

@renkun-ken It seems like in my case there was no .Rprofile in my working directory but I edited Rprofile in C:\Program Files\R\R-3.6.1\library\base\R