feat: 实现for循环语句的解析和执行逻辑，更新词法分析器以支持for关键字

src/ast/stmt.rs

@@ -27,6 +27,14 @@ pub enum Stmt {
         body: Box<Stmt>,
     },
 
+    /// for 循环
+    For {
+        initializer: Option<Box<Stmt>>,
+        condition: Option<Expr>,
+        step: Option<Expr>,
+        body: Box<Stmt>,
+    },
+
     /// 函数声明
     Function {
         name: String,