Further context with additional GitHub Labels

[mbbroberg]

We started pretty light on labels and continue to review them. It's time to dig into it again.

Maintainers need more context when reviewing issues in the backlog and pull requests alike. Our labels do not meet that need today. What's definitely missing:

• ability to quickly see PRs that proposal breaking changes
• more options for how to review an issue's status (wrong repo, flaky test)
• more options to tag an active PR (do not merge, needs 2nd review)

I'll propose a set of additional labels from this Issue that takes other team's practices into account and also meets our needs as a project team.