Add workflow to deploy the sample index (#791)

* Add workflow to build and deploy the sample index

* update gh-pages action

* fix yaml

* create web/ directory in build

* grammar

* add ignored directories

* revert pubspec.lock files

* add job to run _tool/verify_samples.dart

* Update filipino_cuisine for Flutter 2

* remove timeflow demo.

The unnamed List constructor is now deprecated, refactoring
this code to use add() requires more knowledge about the code for
this demo.
https://dart.dev/null-safety/understanding-null-safety#no-unnamed-list-constructor

* update slide_puzzle

* ensure stable channel is used to verify

* move verify web demos action into separate yaml file - avoid running
on each flutter version.

* add on: pull_request

* update slide_puzzle

* Update gh-pages.yml

* Add copyright header

web/samples_index/tool/grind.dart

@@ -19,7 +19,7 @@ void testCli() async => await TestRunner().testAsync(platformSelector: 'vm');
 
 @Task()
 void analyze() {
-  PubApp.local('tuneup')..run(['check']);
+  PubApp.local('tuneup').run(['check']);
 }
 
 @Task('deploy')