Add copyExpandCanvas to Transform Commands, an doc

doc/commands.md

@@ -431,6 +431,11 @@ void copyCropCircle({ int? radius, int? centerX, int? centerY });
 void copyCrop({ required int x, required int y, required int width,
   required int height, num radius = 0 });
 
+void copyExpandCanvas({ required int newWidth, required int newHeight, 
+  ExpandCanvasPosition position = ExpandCanvasPosition.center,
+  Color? backgroundColor, 
+  Image? toImage });
+
 void copyFlip({ required FlipDirection direction });
 
 void copyRectify({ required Point topLeft,

lib/src/command/transform/copy_expand_canvas.dart

@@ -0,0 +1,35 @@
+import '../../../image.dart';
+import '../../transform/copy_expand_canvas.dart';
+
+class CopyExpandCanvasCmd extends Command {
+  int newWidth;
+  int newHeight;
+  ExpandCanvasPosition position;
+  Color? backgroundColor;
+  Image? toImage;
+
+  CopyExpandCanvasCmd(
+    Command? input, {
+    required this.newWidth,
+    required this.newHeight,
+    this.position = ExpandCanvasPosition.center,
+    this.backgroundColor,
+    this.toImage,
+  }) : super(input);
+
+  @override
+  Future<void> executeCommand() async {
+    await input?.execute();
+    final img = input?.outputImage;
+    outputImage = img != null
+        ? copyExpandCanvas(
+            img,
+            newWidth: newWidth,
+            newHeight: newHeight,
+            position: position,
+            backgroundColor: backgroundColor,
+            toImage: toImage,
+          )
+        : null;
+  }
+}