Add workflow to label stale issues and PRs

[mhucka]

As more and more issues and pull requests start to accumulate, it can be helpful to automatically label stale issues and PRs and automatically close them after a time.

This workflow labels issues with stale if the have not had any activity for 90 days or more, and auto-closes them if they have not had any further activity 60 days after that.

[LalehB]

Thank you so much, Mike, for adding this. However, I have a concern about this approach:

• What if there is a long-standing issue that needs time to address? It seems to me such issues could be handled manually.

• That said, I think this is a nice idea for stale PRs.

[mhucka]

• What if there is a long-standing issue that needs time to address? It seems to me such issues could be handled manually.

It's true, sometimes issues do take long. There are several options:

1. If anything changes (a comment, a label added, etc), then the clock is reset. Often for long-standing issues, something changes, so this often takes care of the problem naturally.
2. When an issue is auto-labeled with stale, someone can manually remove the label, and that will reset the clock for another 90 days.
3. There's a list of labels that makes issues & PRs exempt. Currently the labels are help wanted and health. We could add others; also, we could add a label specifically to mean "don't ever mark this stale".

It sounds like a new label might be worth adding. What should it be called? Random ideas off the top of my head: never stale, evergreen, undead (just kidding about that last one), or something else?