Added recommended --prettier flag to include eslint-config-prettier

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing issue: fixes If a tslint.json doesn't extend from tslint-config-prettier, recommend Prettier #2, fixes Make trivia/whitespace (Prettier) rules opt-out #302
• That issue was marked as status: accepting prs

Overview

When --prettier is enabled, eslint-config-prettier will always be added to the list of ESLint plugins.

When --prettier isn't enabled:

• If the output configuration already doesn't enable any formatting rules, it'll extend from eslint-config-prettier.
• Otherwise, a CLI message will suggest running with --prettier.

✨ 90 rules replaced with their ESLint equivalents. ✨

❗ 10 ESLint rules behave differently from their TSLint counterparts ❗
  Check ./tslint-to-eslint-config.log for details.

❌ 1 error thrown. ❌
  Check ./tslint-to-eslint-config.log for details.
️
❓ 1 rule is not known by tslint-to-eslint-config to have an ESLint equivalent. ❓
  The "@typescript-eslint/tslint/config" section of ./.eslintrc.js configures eslint-plugin-tslint to run it in TSLint within ESLint.
  Check ./tslint-to-eslint-config.log for details.

⚡ 7 packages are required for this ESLint configuration. ⚡
  npm install @typescript-eslint/eslint-plugin @typescript-eslint/parser @typescript-eslint/tslint-eslint-plugin eslint eslint-plugin-import eslint-plugin-jsdoc eslint-plugin-prefer-arrow --save-dev

☠ Prettier plugins are missing from your configuration. ☠
  We highly recommend running tslint-to-eslint-config --prettier to disable formatting ESLint rules.
  See https://github/typescript-eslint/tslint-to-eslint-config/docs/FAQs.md#should-i-use-prettier.
️
❓ 1 editor setting is not known by tslint-to-eslint-config to have an ESLint equivalent. ❓
  Check ./tslint-to-eslint-config.log for details.

✅ All is well! ✅

Declares typings in a .d.ts file pending DefinitelyTyped/DefinitelyTyped#44245.

[JoshuaKGoldberg]

Slight cleanup: uniqueFromSources already deduplicates with Array.from(new Set(.