fs: support utimes BigInt

What is the problem this feature will solve?

Current utimes FS functions have a granularity of microseconds, while the stats BigInt functionality has a granularity of nanoseconds.

What is the feature you are proposing to solve the problem?

Adding support for BigInt to the utimes functions to enable nanosecond granularity for setting timestamps.

What alternatives have you considered?

The underlying reason for this issue is that floating point second values only have 24 significant digits, which does not extend to exact nanoseconds. Having stats support BigInt u64 times, while having utimes not creates the inability to exactly set and read the same level of precision. Relying on a BigInt input as a u64 nanoseconds value seems the most sensible way to resolve this to me.

首先定位 filesystem 的 utimes 函数和现有的 stats BigInt 功能。比较它们各自如何表示时间戳精度，并确定相关的平台特定路径。当 BigInt 输入能够设置纳秒级时间戳，并且这些值能够以匹配的精度读回时，即表示完成。