新增于: v12.10.0
仅在将 bigint: true 传递给生成对象的方法时存在。
表示此文件上次修改时间的时间戳，以自 POSIX 纪元以来的纳秒为单位。
【Only present when bigint: true is passed into the method that generates
the object.
The timestamp indicating the last time this file was modified expressed in
nanoseconds since the POSIX Epoch.】