Refactor R code

[ManuelHentschel]

This PR contains some restructuring of the R code used in the extension. Changes are:

• Move function definitions from init.R to their own file vsc.R. This hopefully makes it easier to contribute new functions etc. (in vsc.R) without having to worry about the technical issues solved in init.R.
• Attach tools:vscode after loading default packages. This makes it possible to mask e.g. View without having to mess with the environment of the utils package itself.
• Source R code directly from the installation directory of the extension. Since vscode is guaranteed to have access to its own files and the R process is launched by vscode, I don't think this should cause any read-access issues (please correct me if I'm wrong here)
• Move temporary files (request.log, request.lock) to a temporary directory instead of the home directory.

The last two points imply that the directory ~/.vscode-R is not needed anymore.

Todo:

• Check Rstudio api
• Find solution for self-managed sessions (Maybe we should leave an optional "link file" in ~/.vscode-R? Or expose the path to init.R in the extension files somehow and let the user copy it to their .Rprofile?)

[renkun-ken]

I checked rstudioapi and it works now.

[renkun-ken]

LGTM