[Week 2] Challenge: "Day 12, 13, 14 - Classes, Inheritance/ Protocols, Extensions/ Optionals, nil coalescing"

## 나의 Challenge
> "Classes, Inheritance"
"Protocols, Extensions"
"Optionals, nil coalescing"

## 담당
> @llj72

## Guiding Questions
> Inheritance/Protocols/Extenstions/Optionals/Nil Coalescing은 무엇일까?

## Guiding Activities
> 100 days of SwiftUI 공부하기

## 새로 추가할 노드
> Day12.md
Day13.md
Day14.md