feat: 添加内置函数支持（len, typeof, push, pop），更新相关错误处理和测试用例

examples/builtins.ast

@@ -0,0 +1,21 @@
+// Quick smoke test for new builtins
+print("--- len ---")
+print(len("hello"))
+print(len([1, 2, 3]))
+print(len({a: 1, b: 2, c: 3}))
+
+print("--- typeof ---")
+print(typeof(42))
+print(typeof("hi"))
+print(typeof(true))
+print(typeof(nil))
+print(typeof([1, 2]))
+print(typeof({a: 1}))
+print(typeof(print))
+
+print("--- push/pop ---")
+let arr = [1, 2]
+print("push:", push(arr, 3))
+print("arr:", arr)
+print("pop:", pop(arr))
+print("arr:", arr)