[Rd] Feature request: Change default library path on Windows

[Steve Haroz]

Hello,

I'd like to propose moving the default library install location on Windows from:
%USERPROFILE%/Documents/R
to some other location such as:
%USERPROFILE%/R

For many users the Documents folder is backed up or synchronized.
Installing libraries thrashes Documents, and it causes synchronization
issues with Dropbox (I confirm this one), OneDrive, and users with
Network IT policies.

The vast majority of R users won't touch that folder and don't need it
backed up. And, its contents are not really "documents".

There are many blog posts and websites with people complaining about
it or offering workarounds that involve hand editing setting and
environment files, which reduces R's usability and accessibility.
* https://community.rstudio.com/t/help-regarding-package-installation-renviron-rprofile-r-libs-r-libs-site-and-r-libs-user-oh-my/13888/5
* https://accelebrate.com/library/how-to-articles/r-rstudio-library
* https://community.rstudio.com/t/r-studio-library-installation-directory/30725/2
* https://twitter.com/sharoz/status/1418712098444546057
* https://twitter.com/JoeHilgard/status/1419025358070878210

This change should not interfere with any project environment managers
like renv. It should just change the global default for Windows R
users.
Also, I believe that on Mac it is not in Documents, but it's in the
equivalent of %USERPROFILE%/R.

Thanks,
Steve Haroz