enforce code style via GitHub actions?

[achubaty]

See https://github.com/r-lib/actions/blob/v2-branch/examples/style.yaml for an action we can use to auto-style package code and commit the changes.

I think it's worth implementing across our packages to 1) help my brain read/understand code more easily , and 2) facilitate contributions etc. from others, especially new contributors.

@eliotmcintire if you agree, I'll create PR.