Add Option To Increase Number Of Gradients

[victoreronmosele]

This update adds:

• the option to increase the number of gradient colors in the gradient generator and
• the ability to delete a gradient color by
  • pressing the delete key or
  • clicking on the x icon next to a gradient color stop.

However, the minimum amount of gradient colors is set to 2, so it is not possible to delete more than 2.

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit fb22b58):

https://flutter-gradient-generator--pr68-feat-increase-number-81bw89pg.web.app

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