Flow Layout Supplementary Views

https://developer.apple.com/documentation/uikit/uicollectionviewflowlayout/flow_layout_supplementary_views

"Constants that specify the types of supplementary views that can be presented using a flow layout."

flow layout을 사용해서 나타날 수 있는 보완 뷰의 타입을 구체화하는 상수입니다.