Implement random color generator

[victoreronmosele]

This PR implements a random color generator which is used in the following ways:

• The app loads with two random colors for gradient, by default
• The random button now generates two random colors for gradient

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit e5b441e):

https://flutter-gradient-generator--pr24-feat-random-colors-y3sn0dq8.web.app

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