🛠 Repo: Rework ESLint config: flat config, separate Prettier plugin

[JoshuaKGoldberg]

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

https://github.com/mochajs/mocha/blob/645469e1920e2e86458ff068e74eef2e2915083e/.eslintrc.yml is the repo's current ESLint config. It does a few things that folks in ESLint-land don't generally do anymore:

• Define the config in yml instead of JS - or even better, the new ESLint flat config
• Use semistandard plugin:prettier instead of separating out formatting rules to Prettier
  • feat: add blog post about not needing eslint-config-prettier or eslint-plugin-prettier JoshuaKGoldberg/dot-com#140 is my blog post that goes into this more & has links to other resources

Additional Info

I've been trying to work more with ESLint flat configs to help my work on typescript-eslint around them. This is good timing 😄. Assigning to myself.

The config also doesn't extend from eslint:recommended, but that's a separate issue.