Convert prefer-readonly to @typescript-eslint/prefer-readonly

[cheeZery]

🐛 Bug Report

• tslint-to-eslint-config version: 0.5.0
• ESLint version: 6.8.0
• Node version: 11.10.0

Actual Behavior

TSLint rule prefer-readonly will be converted to ESLint's prefer-readonly (see #181 and #191), but alas that rule doesn't exist in ESLint. As stated here prefer-readonly should be converted to @typescript-eslint/prefer-readonly.

Expected Behavior

TSLint rule prefer-readonly will be converted to @typescript-eslint/eslint-plugin's rule @typescript-eslint/prefer-readonly.

closed with #310 😉

[JoshuaKGoldberg]

Aha, thanks! 😄