Add explicit @MainActor to Button initializer (#236)

* Add explicit `@MainActor` to `Button` initializer

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

* Scope

Sources/SwiftNavigation/ButtonState.swift

@@ -321,6 +321,9 @@ 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, *)
+    #if compiler(>=6)
+      @MainActor
+    #endif
     public init<Action>(_ button: ButtonState<Action>, action: @escaping (Action?) -> Void) {
       self.init(
         role: button.role.map(ButtonRole.init),

Sources/SwiftUINavigation/Alert.swift

@@ -149,6 +149,9 @@
     ///     dismisses the alert, and the action is fed to the `action` closure.
     ///   - handler: A closure that is called with an action from a particular alert button when
     ///     tapped.
+    #if compiler(>=6)
+      @MainActor
+    #endif
     public func alert<Value>(
       _ state: Binding<AlertState<Value>?>,
       action handler: @escaping (Value?) -> Void = { (_: Never?) in }