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/interpreter/interpreter.rs

@@ -64,6 +64,22 @@ impl Interpreter {
                 }
                 Ok(None)
             }
+            Stmt::For { initializer, condition, step, body } => {
+                if let Some(init) = initializer {
+                    self.execute(*init)?;
+                }
+                while let Some(cond) = &condition {
+                    let cond_val = self.evaluate(cond.clone())?;
+                    if !self.is_truthy(&cond_val) {
+                        break;
+                    }
+                    self.execute(*body.clone())?;
+                    if let Some(step) = &step {
+                        self.evaluate(step.clone())?;
+                    }
+                }
+                Ok(None)
+            }
             Stmt::Function { name, params, body } => {
                 let func = Value::Function(Rc::new(Function {
                     params,