`fromdateiso8601` doesn't support the "decimal fraction" component in time values.

Describe the bug The ISO8601 time format supports a decimal fraction attached to the smallest component present; this is passably common for seconds to represent micro- or nano-seconds. Technically this is also applicable to the minute and hour components, though I have never seen that used in the wild.

fromdateiso8601 doesn't handle fractional seconds, which is an annoyance since I have several JSON applications that emit them, and where I'd like to do some date comparisons.

To Reproduce

] jq -nr '"2020-12-11T01:00:52.605001Z" | fromdateiso8601'
jq: error (at <unknown>): date "2020-12-11T01:00:52.605001Z" does not match format "%Y-%m-%dT%H:%M:%SZ"

Expected behavior jq(1) would parse fractional seconds; ideally, it would also carry and emit them, but just ignoring the fraction would be sufficient for my needs.

Environment (please complete the following information):

• macOS 10.15.7 x64
• jq-1.6

Additional context This is definitely an extension on the basic POSIX time functions, so perhaps isn't interesting to jq. I'd hope it was, since they are fairly commonly used, but I understand if this is bounced as "not interested".