R - Docker mod for code-server and openvscode-server

This mod adds a R dev environment to code-server/openvscode-server, to be installed/updated during container start.

In code-server docker arguments, set an environment variable DOCKER_MODS=linuxserver/mods:code-server-r

If adding multiple mods, enter them in an array separated by |, such as DOCKER_MODS=linuxserver/mods:code-server-r|linuxserver/mods:code-server-mod2