重写脚本解释器的实现，提高其可读性。

Serein.Script/Node/FlowControl/ProgramNode.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Serein.Script.Node.FlowControl
+{
+    /// <summary>
+    /// 程序入口
+    /// </summary>
+    public class ProgramNode : ASTNode
+    {
+        /// <summary>
+        /// 程序可执行的语句
+        /// </summary>
+        public List<ASTNode> Statements { get; }
+
+        public ProgramNode(List<ASTNode> statements)
+        {
+            Statements = statements;
+        }
+    }
+
+}