Add ESLint for Code Linting + Enforcing Syntax

[evanugarte]

ESLint helps us set rules for syntax in our code. Examples include enforving the use of single quotes, use camelcase for variable naming and more. Use this guide to get started by adding it to the repository.

• If it asks, use a eslint file with JSON format
• The list of rules we can set are here
• Core-v4's .eslintrc.json