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/.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'