Break code into multiple files.

lib/models/expense.dart

@@ -0,0 +1,18 @@
+import '/models/frequency.dart';
+
+class Expense {
+  final String name;
+  final double cost;
+  final Frequency frequency;
+  final String description;
+
+  const Expense(
+      {required this.name,
+      required this.cost,
+      required this.frequency,
+      required this.description});
+
+  String toString() {
+    return "$name, $cost, ${frequency.title}, $description";
+  }
+}