Add continuous integration to make sure package passes R CMD check (GitHub actions?)

[hturner]

We should use continuous integration to make sure that when changes are made the examples and vignette still work, the documentation is still in sync, etc.

Maybe we can do it using GitHub actions, e.g. the simple workflow suggested here: https://github.com/r-lib/actions/tree/master/examples#quickstart-ci-workflow

[hturner]

Helper functions to use GitHub actions: https://usethis.r-lib.org/reference/github_actions.html