Move files to src and comply with the dart package layout convention (#621)

* 👌 Use `Offset` type directly in `JoystickAction.update` calculations (#631)

* Move files to src and comply with the dart package layout convention

* Fixing widgets example

Co-authored-by: Serge Matveenko <lig@countzero.co>
Co-authored-by: Erick Zanardo <erickzanardoo@gmail.com>

lib/src/extensions/size.dart

@@ -0,0 +1,20 @@
+import 'dart:math';
+import 'dart:ui';
+
+import 'vector2.dart';
+
+export 'dart:ui' show Size;
+
+extension SizeExtension on Size {
+  /// Creates an [Offset] from the [Size]
+  Offset toOffset() => Offset(width, height);
+
+  /// Creates a [Vector2] from the [Size]
+  Vector2 toVector2() => Vector2(width, height);
+
+  /// Creates a [Point] from the [Size]
+  Point toPoint() => Point(width, height);
+
+  /// Creates a [Rect] from the [Size]
+  Rect toRect() => Rect.fromLTWH(0, 0, width, height);
+}