🛠 Repo: Enable eslint-plugin-promise in ESLint config

[Josh Goldberg ✨]

Tooling Suggestion Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest master branch of the repository.
• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I want to provide a PR to resolve this

Overview

eslint-plugin-promise is a devDependency in package.json:

mocha/package.json

Line 97 in 645469e

 "eslint-plugin-promise": "^6.0.1", 

...but not used in the ESLint config.

The plugin is actually useful though. Let's enable from its recommended preset config?

Additional Info

The devDependency is being removed in #5060, but that's just because it's unused. This issue is suggesting adding it back.