Add menuOrder(_:) modifier

[kaascevich]

This allows setting menu items to always appear in the order you write them in for backends that change the sort order depending on where the menu is opened from (currently only UIKit).

[stackotter]

Just some documentation changes. Implementation looks good

[stackotter]

Tested locally and all seems to work well. For future reference;

import DefaultBackend
import SwiftCrossUI

@main
struct TestApp: App {
    @State var fixedOrder = false

    var body: some Scene {
        WindowGroup {
            VStack {
                Menu("Top button") {
                    Button("Item 1") {}
                    Button("Item 2") {}
                    Button("Item 3") {}
                    Button("Item 4") {}
                }.menuOrder(fixedOrder ? .fixed : .priority)

                Spacer()

                Toggle("Use fixed order", isOn: $fixedOrder)

                Spacer()

                Menu("Bottom button") {
                    Button("Item 1") {}
                    Button("Item 2") {}
                    Button("Item 3") {}
                    Button("Item 4") {}
                }.menuOrder(fixedOrder ? .fixed : .priority)
            }.padding()
        }
    }
}