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/ios/.gitignore

@@ -0,0 +1,34 @@
+**/dgph
+*.mode1v3
+*.mode2v3
+*.moved-aside
+*.pbxuser
+*.perspectivev3
+**/*sync/
+.sconsign.dblite
+.tags*
+**/.vagrant/
+**/DerivedData/
+Icon?
+**/Pods/
+**/.symlinks/
+profile
+xcuserdata
+**/.generated/
+Flutter/App.framework
+Flutter/Flutter.framework
+Flutter/Flutter.podspec
+Flutter/Generated.xcconfig
+Flutter/ephemeral/
+Flutter/app.flx
+Flutter/app.zip
+Flutter/flutter_assets/
+Flutter/flutter_export_environment.sh
+ServiceDefinitions.json
+Runner/GeneratedPluginRegistrant.*
+
+# Exceptions to above rules.
+!default.mode1v3
+!default.mode2v3
+!default.pbxuser
+!default.perspectivev3