Add Dependabot config for GitHub Actions

[wadells]

This will help ensure that upstream GitHub actions stay up to date. It operationalizes updates like https://github.com/jetpack-io/devbox-install-action/pull/30 such that they're never missed.

https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

Testing

You can see the PRs that this config will open in my fork here:

https://github.com/wadells/devbox-install-action/pulls?q=+is%3Apr+author%3Aapp%2Fdependabot+

[LucilleH]

@wadells thank you! This is very helpful

[wadells]

Thank you @LucilleH. 🙏