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/macos/Runner/Configs/AppInfo.xcconfig

@@ -0,0 +1,14 @@
+// Application-level settings for the Runner target.
+//
+// This may be replaced with something auto-generated from metadata (e.g., pubspec.yaml) in the
+// future. If not, the values below would default to using the project name when this becomes a
+// 'flutter create' template.
+
+// The application's name. By default this is also the title of the Flutter window.
+PRODUCT_NAME = bookstore
+
+// The application's bundle identifier
+PRODUCT_BUNDLE_IDENTIFIER = dev.flutter.bookstore
+
+// The copyright displayed in application information
+PRODUCT_COPYRIGHT = Copyright © 2021 dev.flutter. All rights reserved.