[dmd-internals] Auto-warn authors followed by auto-close of very stale PRs

[Andrej Mitrovic via dmd-internals]

It seems that after open pulls page 4 or so on the DMD side of things
a pretty large number of PRs are completely broken and haven't been
touched in months, or worse in years. E.g.
https://github.com/dlang/dmd/pull/4658

There is really no benefit to having PRs just sit idly like that.

I think we could automate the clean-up of stale pulls and we could
even do it in stages:

After X amount of time we could make `dlang-bot` attempt to notify the
author of his stale PR, then let some more time pass again, and if the
PR is still in bad shape `dlang-bot` would close the PR, send a
notification to the PR author about the pull being closed, but also CC
to dmd-internals / phobos-internals / etc in case the author went AWOL
so we have a chance of picking up the work from there.

Thoughts?