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/screens/scaffold.dart

@@ -0,0 +1,54 @@
+// 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 'package:adaptive_navigation/adaptive_navigation.dart';
+import 'package:flutter/material.dart';
+
+import '../routing.dart';
+import 'scaffold_body.dart';
+
+class BookstoreScaffold extends StatelessWidget {
+  const BookstoreScaffold({
+    Key? key,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    final routeState = RouteStateScope.of(context)!;
+    final selectedIndex = _getSelectedIndex(routeState.route.pathTemplate);
+
+    return Scaffold(
+      body: AdaptiveNavigationScaffold(
+        selectedIndex: selectedIndex,
+        body: const BookstoreScaffoldBody(),
+        onDestinationSelected: (idx) {
+          if (idx == 0) routeState.go('/books/popular');
+          if (idx == 1) routeState.go('/authors');
+          if (idx == 2) routeState.go('/settings');
+        },
+        destinations: const [
+          AdaptiveScaffoldDestination(
+            title: 'Books',
+            icon: Icons.book,
+          ),
+          AdaptiveScaffoldDestination(
+            title: 'Authors',
+            icon: Icons.person,
+          ),
+          AdaptiveScaffoldDestination(
+            title: 'Settings',
+            icon: Icons.settings,
+          ),
+        ],
+      ),
+    );
+  }
+
+  int _getSelectedIndex(String pathTemplate) {
+    if (pathTemplate.startsWith('/books')) return 0;
+    if (pathTemplate == '/authors') return 1;
+    if (pathTemplate == '/settings') return 2;
+    return 0;
+  }
+}