add form_app (#463)

* add form_app

* revert unrelated changes

* use links in README

* remove unnecessary type annotation

* Remove GlobalKey for sign_in_http sample

* reword comment

* run package:copywriter

* use relative import

* fix quotes

* Add punctuation

* make sure setState() is used in form_widgets sample

* avoid first person pronoun

* store date state in parent widget in form_widgets.dart

experimental/form_app/pubspec.yaml

@@ -0,0 +1,22 @@
+name: form_app
+description: A sample demonstrating different types of forms and best practices
+publish_to: 'none'
+version: 1.0.0+1
+environment:
+  sdk: ">=2.7.0 <3.0.0"
+dependencies:
+  flutter:
+    sdk: flutter
+  cupertino_icons: ^0.1.3
+  intl: any
+  http: ^0.12.0
+  mockito: 4.1.1
+  json_annotation: ^3.0.0
+  english_words: ^3.1.0
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+  json_serializable: ^3.0.0
+  build_runner: ^1.10.0
+flutter:
+  uses-material-design: true