Duration format does not support ISO 8601 fractional values

ISO 8601 allows fractional values in a duration for the trailing component of the duration string. RFC 3339 attempts to define a grammar for ISO 8601 durations in [appendix A](https://datatracker.ietf.org/doc/html/rfc3339#appendix-A) but also notes that ISO 8601 is authoritative. That grammar incorrectly drops support for fractional values.

The ajv format for durations follows the RFC 3339 grammar, so does not currently support the full ISO 8601 spec. Could you please update the duration format to support fractional durations on the trailing component.

I am unable to link the ISO duration spec directly because it is not public, however the [derived Wikipedia page](https://en.wikipedia.org/wiki/ISO_8601#Durations) describes it reasonably well.