feat!: Restructure library

Moves all files into `lib/src` and selectively exports classes and
functions from `lib/stroke_order_animator.dart`. This provides a cleaner
public API.

example/lib/main.dart

@@ -2,9 +2,6 @@ import 'package:flutter/material.dart';
 import 'package:fluttertoast/fluttertoast.dart';
 import 'package:http/http.dart' as http;
 import 'package:provider/provider.dart';
-import 'package:stroke_order_animator/download_stroke_order.dart';
-import 'package:stroke_order_animator/stroke_order.dart';
-import 'package:stroke_order_animator/stroke_order_animation_controller.dart';
 import 'package:stroke_order_animator/stroke_order_animator.dart';
 
 void main() {

lib/src/distance_2_d.dart

@@ -0,0 +1,6 @@
+import 'dart:math';
+import 'dart:ui';
+
+double distance2D(Offset p, Offset q) {
+  return sqrt(pow(p.dx - q.dx, 2) + pow(p.dy - q.dy, 2));
+}

lib/src/quiz_summary.dart

@@ -0,0 +1,20 @@
+import 'dart:ui';
+
+class QuizSummary {
+  QuizSummary(this._nStrokes) {
+    reset();
+  }
+  final int _nStrokes;
+  int get nStrokes => _nStrokes;
+
+  late List<int> mistakes;
+  late List<List<Offset>> correctStrokePaths;
+
+  int get nTotalMistakes =>
+      mistakes.fold(0, (previous, current) => previous + current);
+
+  void reset() {
+    mistakes = List.generate(nStrokes, (index) => 0);
+    correctStrokePaths = List.generate(nStrokes, (index) => []);
+  }
+}

lib/stroke_order.dart → lib/src/stroke_order.dart

@@ -1,7 +1,7 @@
 import 'dart:convert';
 
 import 'package:flutter/rendering.dart';
-import 'package:stroke_order_animator/download_stroke_order.dart';
+import 'package:stroke_order_animator/stroke_order_animator.dart';
 import 'package:svg_path_parser/svg_path_parser.dart';
 
 /// Represents the stroke order of a character.

lib/stroke_order_animation_controller.dart → lib/src/stroke_order_animation_controller.dart

@@ -1,6 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
-import 'package:stroke_order_animator/stroke_order.dart';
+import 'package:stroke_order_animator/src/distance_2_d.dart';
 import 'package:stroke_order_animator/stroke_order_animator.dart';
 
 /// A ChangeNotifier that controls the behavior of a stroke order diagram.
@@ -555,22 +555,3 @@ class StrokeOrderAnimationController extends ChangeNotifier {
     notifyListeners();
   }
 }
-
-class QuizSummary {
-  QuizSummary(this._nStrokes) {
-    reset();
-  }
-  final int _nStrokes;
-  int get nStrokes => _nStrokes;
-
-  late List<int> mistakes;
-  late List<List<Offset>> correctStrokePaths;
-
-  int get nTotalMistakes =>
-      mistakes.fold(0, (previous, current) => previous + current);
-
-  void reset() {
-    mistakes = List.generate(nStrokes, (index) => 0);
-    correctStrokePaths = List.generate(nStrokes, (index) => []);
-  }
-}