Support for macOS?

⚠️ **This is a very large task. Please coordinate here before doing any work.**

ℹ️ I don't have a need for this yet, but might in the future.

### ToDo

Here's a rough, high-level outline of what needs to be done.

- [ ] Figure out the main differences between the `NSCollectionView` infra and `UICollectionView` infra
- [ ] What can we reuse? What cannot be reused?
- [ ] Should macOS just get its own library? Does it make sense to include it here?
- [ ] Tests for everything
- [ ] Add macOS example app to demonstrate, write UI Tests

### Resources

- [Implementing Modern Collection Views](https://developer.apple.com/documentation/uikit/views_and_controls/collection_views/implementing_modern_collection_views)