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/github_dataviz/pubspec.lock

@@ -7,7 +7,7 @@ packages:
       name: characters
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.0-nullsafety.5"
+    version: "1.1.0"
   charcode:
     dependency: transitive
     description:
@@ -21,7 +21,7 @@ packages:
       name: collection
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.15.0-nullsafety.5"
+    version: "1.15.0"
   flutter:
     dependency: "direct main"
     description: flutter
@@ -54,7 +54,7 @@ packages:
       name: meta
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.3.0-nullsafety.6"
+    version: "1.3.0"
   path:
     dependency: transitive
     description:
@@ -101,13 +101,13 @@ packages:
       name: typed_data
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.3.0-nullsafety.5"
+    version: "1.3.0"
   vector_math:
     dependency: transitive
     description:
       name: vector_math
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.0-nullsafety.5"
+    version: "2.1.0"
 sdks:
   dart: ">=2.12.0-0 <3.0.0"