feature: create a FormField implementation for use with flutter_form_builder

[nlfiedler]

Saw this suggested somewhere, so filing it here. Would be convenient if this library provided a FormField implementation for use with the flutter_form_builder library, which is popular and works well. An example can be found in the https://github.com/flutter-form-builder-ecosystem/form_builder_extra_fields repository which also contains a fork of the InputChips widget from the original repo (/danvick/flutter_chips_input).