v7

[lydell]

• Require ESLint v7 or later (needed by improvements below).
• Turn no-tabs into a validatable rule (requires bumping the minimum ESLint version). Closes Update stuff regarding no-tabs #55.
• Only turn off and warn about arrow-body-style and prefer-arrow-callback if eslint-plugin-prettier is used. Requires Turn off problematic rules in recommended config (prepare for next eslint-config-prettier version) eslint-plugin-prettier#360 to be merged as well. People who use eslint-plugin-prettier need to add "prettier/prettier" to their "extends" array, or use an up-to-date plugin:prettier/recommended. Closes Move eslint-plugin-prettier specific rules to its own file #155.
• Improve the CLI:
  • Instead of telling people to pipe eslint --print-config output into the tool, use eslint.calculateConfigForFile added in ESLint v7.
  • Rename from eslint-config-prettier-check to eslint-config-prettier so that npx eslint-config-prettier index.js works regardless of whether eslint-config-prettier already was installed or not.
• Reduce package size by 75% by not including unnecessary stuff in the npm package.