Added M3 demo app (#1189)

* Added M3 demo app

* Changed to use the centralized lint configuration.

* Moved the app at the root level

* Added to CI

* Added app to beta CI and commented app on stable CI file

* Update README.md

* Addressed comments

* fixed comments

* Reverted app name in CI files.

* Added project to the Pub Dependabot config

* Linting pass for current beta channel standards

Co-authored-by: Qun Cheng <quncheng@google.com>
Co-authored-by: Craig Labenz <craig.labenz@gmail.com>

material_3_demo/.gitignore

@@ -0,0 +1,47 @@
+# Miscellaneous
+*.class
+*.log
+*.pyc
+*.swp
+.DS_Store
+.atom/
+.buildlog/
+.history
+.svn/
+migrate_working_dir/
+
+# IntelliJ related
+*.iml
+*.ipr
+*.iws
+.idea/
+
+# The .vscode folder contains launch configuration and tasks you configure in
+# VS Code which you may wish to be included in version control, so this line
+# is commented out by default.
+#.vscode/
+
+# Flutter/Dart/Pub related
+**/doc/api/
+**/ios/Flutter/.last_build_id
+.dart_tool/
+.flutter-plugins
+.flutter-plugins-dependencies
+.packages
+.pub-cache/
+.pub/
+/build/
+
+# Web related
+lib/generated_plugin_registrant.dart
+
+# Symbolication related
+app.*.symbols
+
+# Obfuscation related
+app.*.map.json
+
+# Android Studio will place build artifacts here
+/android/app/debug
+/android/app/profile
+/android/app/release