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/old_snippets/navigator/named_routes.dart

@@ -0,0 +1,68 @@
+// Copyright 2020, 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.
+
+/// Shows how to use define named routes via the `routes` parameter on
+/// MaterialApp, and navigate using Navigator.pushNamed.
+library named_routes;
+
+import 'package:flutter/material.dart';
+
+void main() {
+  runApp(const Nav2App());
+}
+
+class Nav2App extends StatelessWidget {
+  const Nav2App({Key? key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      routes: {
+        '/': (context) => const HomeScreen(),
+        '/details': (context) => const DetailScreen(),
+      },
+    );
+  }
+}
+
+class HomeScreen extends StatelessWidget {
+  const HomeScreen({Key? key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: Center(
+        child: TextButton(
+          child: const Text('View Details'),
+          onPressed: () {
+            Navigator.pushNamed(
+              context,
+              '/details',
+            );
+          },
+        ),
+      ),
+    );
+  }
+}
+
+class DetailScreen extends StatelessWidget {
+  const DetailScreen({Key? key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: Center(
+        child: TextButton(
+          child: const Text('Pop!'),
+          onPressed: () {
+            Navigator.pop(context);
+          },
+        ),
+      ),
+    );
+  }
+}