feat: 添加对象索引访问和赋值支持，更新相关错误处理和测试用例

examples/forin.ast

@@ -5,13 +5,11 @@ for (x in arr) {
     print(x)
 }
 
-print("=== object keys ===")
+print("=== object keys + values ===")
 let obj = {name: "Aster", year: 2025}
-let keys = ""
 for (k in obj) {
-    keys = keys + k + " "
+    print(k + ": " + obj[k])
 }
-print(keys)
 
 print("=== string chars ===")
 for (c in "Hi") {