chore: Add probot

[endocrimes]

This is a first draft at a GitHub Probot configuration for managing those issues
with a long enough cycle time that it shouldn't catch cases where there is
active discussion, and it'll ignore anything we've explicitly called out an
enhancement, bug, or docs change, but will prompt people for a response if
we're waiting-reply etc.

I'm especially looking for feedback on the language usage, because I think the
thresholds are about correct for the volume on this repo.

Link to source/docs: https://github.com/probot/stale

[banks]

Some suggestions but seems good overall!