move experimental/form_app to root of project (#623)

This allows us to reference this sample from other places.

See https://github.com/flutter/flutter/pull/70321 for more context.

form_app/README.md

@@ -0,0 +1,31 @@
+# form_app
+
+A sample demonstrating different types of forms and best practices.
+
+## Sign In w/ HTTP
+[*lib/src/sign_in_http.dart*](lib/src/sign_in_http.dart)
+
+A sign in form using `package:http` to send a request.
+
+## Form widgets
+[*lib/src/form_widgets.dart*](lib/src/form_widgets.dart)
+
+A stylized form that uses widgets like TextField, DatePicker, Slider, Checkbox,
+and Switch.
+
+## Autofill
+[*lib/src/autofill.dart*](lib/src/autofill.dart)
+
+A form that uses AutofillGroup to auto-fill the users name, email, and address.
+
+In order to use Autofill in a browser, your app needs to be hosted with HTTPS.
+If you would like to test locally, you can build the app in release mode
+(`flutter run -d chrome --release --web-port=5000`) and use
+[ngrok](https://ngrok.com/) to create an HTTPS url for your local app (`ngrok
+http 5000`)
+
+## Validation
+[*lib/src/validation.dart*](lib/src/validation.dart)
+
+A form that alerts the user if the data entered is invalid.
+