Reorganise GitHub Actions

I'd like to define workflows here that I see are present, so we can clean up the current CI we have going on.

For routine maintenance

1. Generate bindings from new versions of R, R-devel, etc.
2. `nonAPI.txt` also changes with these versions, and there is an action for it

For PR authors (contributors outside & inside the org)

3. Run tests, layout tests, on the generated bindings due to their changes, that they have not seen before, because they don't have all platforms available to them, nor all R versions installed.
4. Append their PRs with generated bindings, similar to (1), but should refer to current PR and push to current PR branch.

For maintainers / ambitious PR authors:

5. Run `extendr` main on PR'd generated bindings, I believe this is what is meant with #162