Stale Bot
Adds a stale bot based on GitHub Actions.

This bot is slightly different than the previous one in that it comments that it is closing the issue and closes it immediately, rather than waiting 7 days. From what I've seen up to now, this shouldn't be an issue at all.

I've commented out the code for now so that it doesn't actually close issues, but you can take a look at which would be closed [here](https://github.com/huggingface/transformers/runs/2027394567?check_suite_focus=true). Will un-comment the code before merging.
The `GITHUB_TOKEN` got rate-limited, unfortunately. The PR should be good to go, I'll try to run it again tomorrow night.