fix: #1396 fix dynamic fields example widget issues

[ArturAssisComp]

Connection with issue(s)

Close #1396

Connected to #???

Solution description

Handle the list of text fields in such a way that each element is deleted based on its unique local key, not based on the size of the list.

Screenshots or Videos

https://github.com/flutter-form-builder-ecosystem/flutter_form_builder/assets/60040236/d173d978-febe-4b6b-85d5-c35973ec205f

To Do

• [X] Read contributing guide
• [X] Check the original issue to confirm it is fully satisfied
• [X] Add solution description to help guide reviewers
• [ ] Add unit test to verify new or fixed behaviour
• [X] If apply, add documentation to code properties and package readme

[ArturAssisComp]

I did not add unit tests because there is no unit tests in the folder example. But, if you want, I can add.

[deandreamatias]

Thanks for your contribution!