Simplify OverviewMap usage

[mhdostal]

It would be great if adding an overview map were simpler. One way of doing that would be to have a custom view modifier for the overview map in the toolkit:

MapView(map: map)
    .overviewMap(alignment: .topTrailing)

A proof of concept is as follows:

extension MapView {
    func overviewMap(alignment: Alignment) -> some View {
        MapViewWithOverviewMap(mapView: self, alignment: alignment)
    }
}

struct MapViewWithOverviewMap: View {
    let mapView: MapView
    let alignment: Alignment
    
    @State private var viewpoint: Viewpoint?
    @State private var visibleArea: Polygon?
    
    var body: some View {
        mapView
            .onViewpointChanged(kind: .centerAndScale) { viewpoint = $0 }
            .onVisibleAreaChanged { visibleArea = $0 }
            .overlay(alignment: alignment) {
                OverviewMap.forMapView(
                    with: viewpoint,
                    visibleArea: visibleArea
                )
                .frame(width: 200, height: 132)
                .padding()
            }
    }
}

As a proof of concept, it isn't without problems. We would need to change how those modifiers on map/scene view are implemented since users may also want to use them themselves (currently each subsequent use would override the previous).

The overview map could be customized with an overviewMapStyle(_:) modifiers, similar to other SwiftUI views. This should be explored as part of this issue; the OverviewMap already provides modifiers for symbol, map, and scaleFactor. Should they be combined into one overviewMapStyle(map:, symbol:, scaleFactor:)?

To get around the .onViewpointChanged(kind:)/.onVisibleAreaChanged() issue, maybe the .overviewMap(alignment:) modifier could take an optional binding to a Viewpoint and visible area Polygon and then update it alongside the local viewpoint and visible area.

Thanks to @philium for the suggestion!