Enables various packages to be automatically built from their GitHub sources, to facilitate installation via install.packages().
Currently, we build from our development branches.
See https://ropensci.org/blog/2021/06/22/setup-runiverse/ for setup information.
To add a package, simply add its GitHub repo to packages.R, and run that script.
This will generate the packages.json file that r-universe needs to configure package builds.
NOTE: Initially, do not specify a branch, as it needs to be built from main/master successfully first.
Once the newly added package has been built (usually within the hour), it's usually OK to add the branch information and rebuild the packages.json file.