RefreshableScrollView

Refreshable scroll view for SwiftUI.

Currently, iOS 15 allows for a refreshable List, but not a scroll view.
So I’ve added this. It also enables us to customize our animations, etc

An improvement I would make is to add more customization at the call site. However, I figure most apps would like a consistent pull to refresh experience. Up to you. Make changes to the code as you see fit, or just use it as inspiration.