Make NavigationSheet independent of NavigatorObserver

[fujidaiti]

The current implementation of NavigationSheet relies on NavigatorObserver to detect page transitions within the nested navigator. However, this approach has several issues that are difficult to resolve, especially when using go_router (#170, #152). Making NavigationSheet independent of NavigatorObserver will not only solve these problems but also simplify its usage, as future APIs will eliminate the need for developers to manage a NavigatorObserver object.