temporarily use site library to set up VSCode-R ext

[hturner]

The VSCode-R extension checks for the jsonlite and rlang packages during initialization, so that the R Session Watcher will work. We install these packages when we install languageserver in the release version R that we add to the Docker container.

Although these packages only need to be available to the version of R used for background processes (defined by Rpath: Linux setting), the check is run on the version of R used in interactive sessions (defined by Rterm: Linux setting).

This PR adds an .Rprofile to temporarily add the library where the packages are installed to the library search path.

I have also set the options for the PNG graphics to a more sensible size - this may later be replaced by using httpgd graphics (#100 (comment)).

[hturner]

Updated to use httpgd graphics. This requires the httpgd package to be available to the active R process. So now the .Rprofile adds the site library which has the necessary packages for full capability of the VS-Code extension. This library is separate from the library where the base and recommended packages are installed, so there is no clash. E.g. an R build without the recommended packages will have the base R packages built from the source code in the default library, no recommended packages, plus the add-on packages required for VSCode built for the latest R release in the site library.

Only tools:vscode is attached to the search path, which is similar to the RStudio setup. Vanilla R can always be run by running bin/R from the build directory in a terminal, where plots will be sent to Rplots.pdf.