Replace navigation_and_routing with a new sample (#832)

* move snippets into old_snippets directory

* add new navigation_and_routing sample

* add copyright headers

* Apply #827 to old_snippets/ directory and upgrade them to null safety

* Code review comments

- Move Guard class into parser.dart
- Move usage of guards from Delegate to RouteInformationParser
- Rename delegate to SimpleRouterDelegate

* clean up imports

* refactor settings screen, fix bug

* avoid conflicting paths /books/new and /books/1 - rename to book/1

* dispose fields in _BookstoreState class

* remove /books path

This was causing problems

* add comment

* Change BookstoreAuthScope and BookstoreAuthScope to InheritedNotifier

* fix warnings

* Make the initial route configurable, set to '/signin'

* Enable deep linking

https://flutter.dev/docs/development/ui/navigation/deep-linking

* use path URL strategy on the web.

* remove TODO, add comment

navigation_and_routing/lib/src/routing/delegate.dart

@@ -0,0 +1,52 @@
+// Copyright 2021, the Flutter project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/widgets.dart';
+
+import 'parsed_route.dart';
+import 'route_state.dart';
+
+class SimpleRouterDelegate extends RouterDelegate<ParsedRoute>
+    with ChangeNotifier, PopNavigatorRouterDelegateMixin<ParsedRoute> {
+  final RouteState routeState;
+  final WidgetBuilder builder;
+
+  @override
+  final GlobalKey<NavigatorState> navigatorKey;
+
+  SimpleRouterDelegate({
+    required this.routeState,
+    required this.builder,
+    required this.navigatorKey,
+    // ignore: prefer_initializing_formals
+  }) {
+    routeState.addListener(notifyListeners);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return builder(context);
+  }
+
+  @override
+  Future<void> setNewRoutePath(ParsedRoute configuration) async {
+    routeState.route = configuration;
+    return SynchronousFuture(null);
+  }
+
+  @override
+  ParsedRoute get currentConfiguration {
+    return routeState.route;
+  }
+
+  @override
+  void dispose() {
+    routeState.removeListener(notifyListeners);
+    routeState.dispose();
+    super.dispose();
+  }
+}