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/.metadata

@@ -0,0 +1,42 @@
+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled.
+
+version:
+  revision: 7e2d09f29b2f416b63fd6c4544a40d3aaa1ec423
+  channel: master
+
+project_type: app
+
+# Tracks metadata for the flutter migrate command
+migration:
+  platforms:
+    - platform: root
+      create_revision: 7e2d09f29b2f416b63fd6c4544a40d3aaa1ec423
+      base_revision: 7e2d09f29b2f416b63fd6c4544a40d3aaa1ec423
+    - platform: android
+      create_revision: 7e2d09f29b2f416b63fd6c4544a40d3aaa1ec423
+      base_revision: 7e2d09f29b2f416b63fd6c4544a40d3aaa1ec423
+    - platform: ios
+      create_revision: 7e2d09f29b2f416b63fd6c4544a40d3aaa1ec423
+      base_revision: 7e2d09f29b2f416b63fd6c4544a40d3aaa1ec423
+    - platform: macos
+      create_revision: 7e2d09f29b2f416b63fd6c4544a40d3aaa1ec423
+      base_revision: 7e2d09f29b2f416b63fd6c4544a40d3aaa1ec423
+    - platform: web
+      create_revision: 7e2d09f29b2f416b63fd6c4544a40d3aaa1ec423
+      base_revision: 7e2d09f29b2f416b63fd6c4544a40d3aaa1ec423
+    - platform: windows
+      create_revision: 7e2d09f29b2f416b63fd6c4544a40d3aaa1ec423
+      base_revision: 7e2d09f29b2f416b63fd6c4544a40d3aaa1ec423
+
+  # User provided section
+
+  # List of Local paths (relative to this file) that should be
+  # ignored by the migrate tool.
+  #
+  # Files that are not part of the templates will be ignored by default.
+  unmanaged_files:
+    - 'lib/main.dart'
+    - 'ios/Runner.xcodeproj/project.pbxproj'