Split ESLint Config out into separate repository

[kadamwhite]

• Repository created for ESLint config (created as humanmade/eslint-config
• This repository is mirrored to the new one
• PHPCS and Stylelint files are removed from new repository
• Documentation is updated in the new repository
• PRs and issues worth keeping are mirrored to new repository
• ESLint-related code is removed from this repository
• ESLint-related issues and PRs are closed on this repository