refactor(chess): 重构象棋程序基础结构,使用Nullable重构核心代码

- 更新了多个文件的代码结构和类型定义，提高了代码的健壮性和可维护性
- 优化了事件处理、棋子管理和移动逻辑，为后续功能扩展打下坚实基础
- 修复了一些潜在的空指针异常问题，提高了程序的稳定性
- 修复部分bug

Scripts/Src/IBoard.cs

@@ -6,15 +6,15 @@ public interface IBoard {
     public int Cols { get; }
 
     public class SetPieceEventArgs : EventArgs {
-        public IPiece OldPiece { get; set; }
-        public IPiece NewPiece { get; set; }
-        public Vector2I Pos { get; set; }
+        public IPiece? OldPiece { get; set; }
+        public IPiece? NewPiece { get; set; }
+        public Vector2I Pos { get; set; } = new();
     }
 
     public class MoveEventArgs : EventArgs {
-        public IPiece Piece { get; set; }
-        public Vector2I From { get; set; }
-        public Vector2I To { get; set; }
+        public IPiece? Piece { get; set; }
+        public Vector2I From { get; set; } = new();
+        public Vector2I To { get; set; } = new();
     }
 
     event EventHandler<IPiece> OnInsert;
@@ -23,10 +23,10 @@ public interface IBoard {
     event EventHandler<SetPieceEventArgs> OnSetPiece;
 
     bool IsPosOutOfRange(Vector2I pos);
-    IPiece GetPiece(Vector2I pos);
-    IPiece SetPiece(IPiece piece, Vector2I pos);
+    IPiece? GetPiece(Vector2I pos);
+    IPiece? SetPiece(IPiece piece, Vector2I pos);
     bool InsertPiece(IPiece piece, Vector2I pos);
     bool MovePiece(Vector2I from, Vector2I to);
-    IPiece RemovePiece(Vector2I pos);
+    IPiece? RemovePiece(Vector2I pos);
     void Clear();
 }