Is it possible to support `stale.yaml` in addition to `stale.yml`?

[kenji-miyake]

Currently, it seems only stale.yml is supported.

stale/index.js

Line 54 in 10840ea

let config = await getConfig(context, 'stale.yml')

However, since the .yaml extension is officially recommended, it would be nice if .yaml can also be supported.

https://yaml.org/faq.html says:

Is there an official extension for YAML files?
Please use ".yaml" when possible.