make determining what is a PR branch CI provider agnostic

[devinburnette]

right now this only works with CircleCI. maybe we introduce some type of config where we can set a "trunk" or "default branch" pattern, and everything else is a PR?

[samandmoore]

Looking at the code, it's probably not too hard to make it so folks can configure a lambda for it as a config setting. The default can just be the presence of that circleci env var but then someone can override it to be whatever they need, like checking the GitHub event name in GitHub actions or something.