Step 1: Show the Welcome screen

The SignUpForm class is a stateful widget.
This simply means that the widget stores information
that can change, such as user input, or data from a feed.
Since widgets themselves are immutable
(can’t be modified once created),
Flutter stores state information in a companion class,
called the State class. In this lab,
all of your edits will be made to the private
_SignUpFormState class.

Fun fact

The Dart compiler enforces privacy for any identifier
prefixed with an underscore. For more information,
see the Effective Dart Style Guide.

First, in your lib/main.dart file,
add the following class definition for the
WelcomeScreen widget after the SignUpScreen class:

Next, you will enable the button to display the screen
and create a method to display it.