Make Sections Collapsible

[victoreronmosele]

This PR adds the ability to collapse the different sections of the gradient generator.

Screen recordings:

• Expanding and collapsing sections in landscape mode

  https://github.com/victoreronmosele/flutter_gradient_generator/assets/19398044/6bae1c22-ae6b-46bd-8875-2f80b70c741d

• Expanding and collapsing sections in portrait mode

  https://github.com/victoreronmosele/flutter_gradient_generator/assets/19398044/5e139395-6494-42df-bd05-76348cedac43

Closes #64

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Visit the preview URL for this PR (updated for commit a108a8c):

https://flutter-gradient-generator--pr66-feat-collapsible-sec-40eqozz5.web.app

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