Add explicit @MainActor to Button initializer

This works around a regression in behavior introduced by Xcode 16 beta
3.

Sources/SwiftNavigation/ButtonState.swift

@@ -321,6 +321,7 @@ extension ButtonState: Sendable where Action: Sendable {}
     ///   - button: Button state.
     ///   - action: An action closure that is invoked when the button is tapped.
     @available(iOS 15, macOS 12, tvOS 15, watchOS 8, *)
+    @MainActor
     public init<Action>(_ button: ButtonState<Action>, action: @escaping (Action?) -> Void) {
       self.init(
         role: button.role.map(ButtonRole.init),