feat: reject non-calendar ISO strings

[stelardn]

Fixes #2733

This adds a stricter validation to string().isoDate(), rejecting dates that do not exist in the calendar.

It does so by:

• Validating that the day component of the ISO string matches the day derived from the corresponding JavaScript Date.
• Handling edge cases where the difference between days is caused by T24:00 notation, in order to keep support for valid T24:00 ISO strings.
• Preventing invalid dates such as April 31st from being accepted, even when combined with T24:00.