重写脚本解释器的实现，提高其可读性。

Serein.Script/Node/FlowControl/IfNode.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Serein.Script.Node.FlowControl
+{
+    /// <summary>
+    /// 条件节点
+    /// </summary>
+    public class IfNode : ASTNode
+    {
+        /// <summary>
+        /// 条件来源
+        /// </summary>
+        public ASTNode Condition { get; }
+
+        /// <summary>
+        /// 条件为 true 时所执行的语句
+        /// </summary>
+        public List<ASTNode> TrueBranch { get; }
+
+        /// <summary>
+        /// 条件为 false 时所执行的语句
+        /// </summary>
+        public List<ASTNode> FalseBranch { get; }
+        public IfNode(ASTNode condition, List<ASTNode> trueBranch, List<ASTNode> falseBranch)
+            => (Condition, TrueBranch, FalseBranch) = (condition, trueBranch, falseBranch);
+    }
+
+}

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;
+        }
+    }
+
+}

Serein.Script/Node/FlowControl/ReturnNode.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Serein.Script.Node.FlowControl
+{
+    /// <summary>
+    /// 返回值
+    /// </summary>
+    public class ReturnNode : ASTNode
+    {
+        /// <summary>
+        /// 返回值来源
+        /// </summary>
+        public ASTNode Value { get; }
+
+        public ReturnNode(ASTNode returnNode)
+        {
+            Value = returnNode;
+        }
+        public ReturnNode()
+        {
+        }
+    }
+
+}

Serein.Script/Node/FlowControl/WhileNode.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Serein.Script.Node.FlowControl
+{
+    /// <summary>
+    /// 循环条件节点
+    /// </summary>
+    public class WhileNode : ASTNode
+    {
+        /// <summary>
+        /// 循环条件值来源
+        /// </summary>
+        public ASTNode Condition { get; }
+
+        /// <summary>
+        /// 循环中语句块
+        /// </summary>
+        public List<ASTNode> Body { get; }
+        public WhileNode(ASTNode condition, List<ASTNode> body) => (Condition, Body) = (condition, body);
+    }
+
+}