Add ESLint

[artnc]

Main new features, many of which are already enforced in our duolingo.com repo:

• Sort object keys alphabetically
• Collapse small object literals onto one line
• Sort members within each import statement
• Prefer { foo } instead of { foo: foo }
• Prefer returning expression from arrow function instead of using block form
• Name catch variables ex and omit if unused
• Swap if and else branches to avoid unnecessary !
• Use Record instead of { []: } form
• Remove unnecessary async
• Collapse short JSDoc onto one line