New M3 Demo App for Master Channel (#1378)

* Added components

* Added test

* Added README

* Run flutter format

* Added all the updated components and tests

* Added README file

* Update README.md

* Add textfields to M3 demo app

* move textfield to exprerimental/

* Update component_screen_test.dart

* Fixed comment: changed sdk version

* Changed all double quotes to single

* Added trailing comma, changed all private fields to public except private state classes

* Added enum for screen selection

* Removed comment for material_3_demo

* Added experimental m3 demo app to master CI pipeline

* fixed failing tests

* Modified the analysis_options.yaml

* Fixed warning

* Ran flutter format command

Co-authored-by: Qun Cheng <quncheng@google.com>
Co-authored-by: hangyu <108393416+hangyujin@users.noreply.github.com>

experimental/material_3_demo/pubspec.yaml

@@ -0,0 +1,26 @@
+name: material_3_demo
+description:
+  A Flutter project showcasing supported Material 3 components, typography, color system and elevation.
+  Supports different light/dark mode, color seed, and comparison to Material 2.
+
+publish_to: "none"
+
+version: 1.0.0+1
+
+environment:
+  sdk: ">=2.18.0-0 <3.0.0"
+
+dependencies:
+  flutter:
+    sdk: flutter
+
+  cupertino_icons: ^1.0.2
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+
+  flutter_lints: ^2.0.1
+
+flutter:
+  uses-material-design: true