chore: transfer ValueTransformer typdef from form_builder to form_builder_field

Author: danvick

lib/src/form_builder.dart

@@ -1,8 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_form_builder/flutter_form_builder.dart';
 
-typedef ValueTransformer<T> = dynamic Function(T value);
-
 /// A container for form fields.
 class FormBuilder extends StatefulWidget {
   /// Called when one of the form fields changes.

lib/src/form_builder_field.dart

@@ -5,6 +5,8 @@ import 'package:flutter_form_builder/flutter_form_builder.dart';
 enum OptionsOrientation { horizontal, vertical, wrap }
 enum ControlAffinity { leading, trailing }
 
+typedef ValueTransformer<T> = dynamic Function(T value);
+
 /// A single form field.
 ///
 /// This widget maintains the current state of the form field, so that updates