[php-src] Issue #14668: Add support for formatting and creating dates from format for the RFC 9557 format

Issue: https://github.com/php/php-src/issues/14668 Author: stof ### Description [RFC 9557](https://datatracker.ietf.org/doc/rfc9557/) defines an extension of the RFC 3339 format. As this format is used as the string representation for the [proposed JS Temporal API](https://tc39.es/proposal-temporal/) (currently being implemented by the major browsers, so likely to ship in a few months), it would be great to be able to use this format in PHP in order to interoperate with JS. The format used in the Temporal API is described at https://tc39.es/proposal-temporal/docs/#string-persistence-parsing-and-formatting As DatetimeImmutable does not support changing the calendar in PHP, and the default calendar in the JS Temporal API is the ISO 8601 calendar, I think the calendar extension can be omitted. However, I would expect this formatting to include the timezone extension when the DatetimeImmutable being formatted is associated with a IANA timezone (i.e. a DateTimeZone of type 3). When creating a date from a RFC 9557 format, I would expect PHP to read the timezone extension to assign the appropriate timezone in the class using a IANA timezone. There are 2 cases to handle here: - RFC 9557 redefines the meaning of the Z suffix, which has an impact when combining it with a timezone extension (the RFC 3339 date should be considered as being the value in UTC, and then get converted to the IANA timezone, as documented by the example in section 3.3 of the RFC) - if the string defines both a timezone offset and a timezone name, they might be inconsistent. The way to handle that is defined in the RFC: - if the timezone extension is elective, it can be ignored, treating it as if it were not there (and so parsing only the timezone offset, producing a DatetimeImmutable with a DateTimeZone of type 1) - if the timezone extension is critical (indicated by a ! following the opening bracked), the processing must, which should be reported as an error in PHP (like other invalid dates).