Add Dangermattic configuration

[iangmaia]

Description

This PR adds Dangermattic to this repo.
My main goal was to add RuboCop, but I added some other basic checks as well.
Please let me know if it makes sense to add more checks seen in other repos (reference)

Testing Details

The main validation is to make sure CI is 🟢.

To run Danger yourself:

1. Checkout this branch
2. Run bundle install
3. Run:

DANGER_GITHUB_API_TOKEN=<github_token> bundle exec danger pr <PR_URL>

You should get in the console the same errors / warnings reported in the PR.

---

• Please check here if your pull request includes additional test coverage.
• I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[mokagio]

The second action run passed but @dangermattic was not able to report.

I added it as a collaborator to the repo, and verified it can report.