We've switched all of the other repos to use main as the primary branch, so it's time to move this repo as well. Now that GitHub has automatic branch renaming there are a lot fewer issues to deal with. As best I can tell, these are the steps we need to follow:

• Update GitHub action workflow files to use "main" instead of "master" (maybe start with listing both to avoid breaking coverage?)
• Update Makefile.js to replace instances of "master" with "main"
• Update "Working with Rules" documentation to replace instances of "master" with "main"
• Use GitHub branch rename feature to change "master" to "main"

There don't appear to be any other important instances of "master" in the codebase. I believe URLs referring to blobs in the repo will automatically redirect once changed through the GitHub feature.