time_point_precision() extracts the precision from a time point, such
as a sys-time or naive-time. It returns the precision as a single string.
time_point_precision(x)A single string holding the precision of the time point.
time_point_precision(sys_time_now())
#> [1] "nanosecond"
time_point_precision(as_naive_time(duration_days(1)))
#> [1] "day"