Accessibility + visionOS: Observe head anchor system setting

HeadAnchoredView.swift

import SwiftUI

/// An example view that listens to the user's system settings
/// about preferring head-anchored views
struct HeadAnchoredView: View {
    @Environment(\.accessibilityPrefersHeadAnchorAlternative)
    private var accessibilityPrefersHeadAnchorAlternative

    var body: some View {
        if accessibilityPrefersHeadAnchorAlternative {
            Text("User does NOT want things head anchored.")
        } else {
            Text("User is ok with things that are head anchored. Provide an alternative.")
        }
    }
}