Add branch protection to `main`

[partylikeits1983]

We should add branch protection to the main branch. This will mirror other repositories within the org. This would prevent merges to main without a PR review approval.

I've accidentally pushed to main in the past, which if there was a branch protection, this would have been reverted.

This would prevent accidental pushes to main and merging of PRs without approvals.

[mmagician]

This should be an org-wide branch protection rule

[huitseeker]

Tooling for this: https://github.com/genuinetools/pepper