Allow configure nanosecond's precision

[tisonkun]

Background - fast/logforth#40 (comment)

Currently, the default formatter can cause unaligned nanosecond:

2024-08-11T13:47:37.947497+00:00[UTC] ERROR simple_stdio: examples/simple_stdio.rs:32 Hello error!
2024-08-11T13:47:37.98353+00:00[UTC]  WARN simple_stdio: examples/simple_stdio.rs:33 Hello warn!
2024-08-11T13:47:37.983557+00:00[UTC]  INFO simple_stdio: examples/simple_stdio.rs:34 Hello info!
2024-08-11T13:47:37.983579+00:00[UTC] DEBUG simple_stdio: examples/simple_stdio.rs:35 Hello debug!
2024-08-11T13:47:37.983602+00:00[UTC] TRACE simple_stdio: examples/simple_stdio.rs:36 Hello trace!

@BurntSushi suggests we can make it configurable at both:

• jiff::fmt::temporal::DateTimePrinter
• https://doc.rust-lang.org/std/fmt/struct.Formatter.html#method.precision

Record for any improvement chance.

[tisonkun]

Used in fast/logforth#58.