[React] React 기본 문법

https://violetboralee.medium.com/react%EB%A5%BC-%EB%B0%B0%EC%9A%B0%EA%B8%B0-%EC%A0%84%EC%97%90-%EC%95%8C%EC%95%84%EC%95%BC-%ED%95%A0-javascript%EA%B8%B0%EC%B4%88-e0665f8cbee0