feat(#451): basic free text question data model

core/lib/src/models/questionnaire/question.dart

@@ -13,6 +13,7 @@ abstract class Question<V> {
     ScaleQuestion.questionType: (json) => ScaleQuestion.fromJson(json),
     AnnotatedScaleQuestion.questionType: (json) => AnnotatedScaleQuestion.fromJson(json),
     VisualAnalogueQuestion.questionType: (json) => VisualAnalogueQuestion.fromJson(json),
+    FreeTextQuestion.questionType: (json) => FreeTextQuestion.fromJson(json),
   };
   static const String keyType = 'type';
   String type;

core/lib/src/models/questionnaire/questions/free_text_question.dart

@@ -0,0 +1,20 @@
+import 'package:json_annotation/json_annotation.dart';
+
+import 'package:studyu_core/core.dart';
+
+part 'free_text_question.g.dart';
+
+@JsonSerializable()
+class FreeTextQuestion extends Question<String> {
+  static const String questionType = 'freeText';
+
+  FreeTextQuestion(): super(questionType);
+
+  FreeTextQuestion.withId(): super.withId(questionType);
+
+  factory FreeTextQuestion.fromJson(Map<String, dynamic> json) => _$FreeTextQuestionFromJson(json);
+  @override
+  Map<String, dynamic> toJson() => _$FreeTextQuestionToJson(this);
+
+  Answer<String> constructAnswer(String response) => Answer.forQuestion(this, response);
+}

core/lib/src/models/questionnaire/questions/questions.dart

@@ -1,6 +1,7 @@
 export 'annotated_scale_question.dart';
 export 'boolean_question.dart';
 export 'choice_question.dart';
+export 'free_text_question.dart';
 export 'scale_question.dart';
 export 'slider_question.dart';
 export 'visual_analogue_question.dart';