Create PaginatingLazyVStack as a generic scrollable list container

[amy-at-kickstarter]

📲 What

Add PaginatingLazyVStack to our component library.

🤔 Why

As part of SwiftUI work, I'm working on re-writing the old backer dashboard using SwiftUI. (Even if it never ships, I think the work will be educational.)

The main component of this page is going to be a scrolling list of cards. That seems like fairly generic functionality, so I wrote this paginating/refreshable VStack as a generic implementation of it.

👀 See

Here's what the example view looks like - it supports infinite scroll and pull-to-refresh.