FFI for creating datetime types from nanoseconds

[linusg]

#359 made sense for V8 given it only uses a time source with millisecond resolution: https://github.com/v8/v8/blob/d67f761c7ecec6b5a88aa620c944074cf2138ce1/src/objects/js-temporal-objects.cc#L2197-L2211

For Kiesel this is currently lossy: https://codeberg.org/kiesel-js/kiesel/src/commit/9793dd117ed4bc924400a113de54979bb682d204/src/builtins/temporal/now.zig#L112-L116

> Temporal.Now.plainDateTimeISO()
Temporal.PlainDateTime(2025, 9, 16, 20, 57, 43, 352, 0, 0, "iso8601")
> Temporal.Now.zonedDateTimeISO().toPlainDateTime()
Temporal.PlainDateTime(2025, 9, 16, 20, 57, 44, 326, 52, 73, "iso8601")
>