Enable branch protection

[dajmcdon]

• Suggest we protect main to require a PR with 1 review and passing checks.
• Possibly @gvegayon as CODEOWNER?

[gvegayon]

Good idea, @dajmcdon; I would suggest you, @annecori, and @cmilando as well, at least for the R bits, as you know better what's going on with the actual packages.