Improve session watcher initialization

[renkun-ken]

Closes #196.
Related to #177, #179.

In this PR:

• Use USERPROFILE for Windows according to os: add homedir() nodejs/node#1791: https://github.com/nodejs/node/pull/1791/files#diff-d9bb084c9b6137788ddd0314b2e4df6dR11.
• Clarify when a local .Rprofile exists.
• Use R_PROFILE_USER as @randy3k suggests so that there's no need for manual configuration if users starts R session using R: Create R Terminal command.
• init.R checks TERM_PROGRAM=vscode so that it is only effective in VSCode terminal.
• Update README.md to cover basic usage, advanced usage and how to disable session watcher.

[andycraig]

Thank you for switching it to USERPROFILE!

[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.

# ~/.vscode-R/.Rprofile

if (file.exists(".Rprofile")) {
	source(".Rprofile")
} else if (file.exists(normalizePath("~/.Rprofile"))) {
	source("~/.Rprofile")
}

if (<init.R has not been execuated>) {
	source(file.path(Sys.getenv(if (.Platform$OS.type == "windows") "USERPROFILE" else "HOME"), ".vscode-R", "init.R"))
}

This approach would require zero configuration for R sessions launched by vscode-R. Only if a user wants to use session watcher in external R processes, they would have to edit the .Rprofile.

[renkun-ken]

Thanks @randy3k for your great suggestion. With this PR, user won't have to do any manual configuration if user only starts R session via R: Create R Terminal command.

Now, only users who want to use R session watcher with self-managed R session will have to do some extra configuration. Let me update the README to reflect all these changes.

[andycraig]

@renkun-ken This PR is now conflicted with master. Would you be able to resolve that and then I'll merge it? Thanks!

[renkun-ken]

@andycraig Conflicts resolved. Thanks!

[andycraig]

@renkun-ken Great, thanks again!