…otationView`.

This uses `anchorPoint` from `UIView` when available (iOS 16+), otherwise it relies on `MKAnnotationView` `centerOffset` and converts the `anchorPoint` percentages into pixel values.

After applying the frame fix in the previous commit, the annotations displayed on the map with a white background. This brings back the clear background they previously had.

`UnitPoint` - https://developer.apple.com/documentation/swiftui/unitpoint - is more SwiftUI-y than `CGPoint`, and will allow the anchorPoint to use the constants like `.center` and `.bottom`.

Extract extensions to make it trivial to convert the `UnitPoint` to a `centerOffset` within a rect, and to convert back to a `CGPoint` for the `UIView` `anchorPoint` property.

The `#available(iOS 16)` wasn't preventing the `anchorPoint` from being set, but that is only available on `UIView`, not `NSView`. Ensuring `UIKit` is available fixes the error, and we fall back to `centerOffset` otherwise.