feat: add xsd:datetime codec and data type

[mark-dropbear]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @mark-dropbear, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the library's capability by adding full support for the xsd:dateTime data type. It introduces a custom XsdDateTime class designed to correctly interpret and manage date-time values according to XSD specifications, particularly concerning timezone information and 'floating' date-times. This ensures that date-time values can be accurately parsed from and serialized to XSD strings, maintaining data integrity across operations.

Highlights

• New xsd:datetime Codec and Data Type: Introduced a new XsdDateTime class and its corresponding codec (XsdDateTimeCodec) to handle xsd:dateTime values, ensuring XSD-compliant parsing and serialization.
• Timezone Preservation: The XsdDateTime class specifically addresses the challenge of 'floating' (timezone-less) xsd:dateTime values and preserves the original timezone offset for accurate round-tripping, which Dart's native DateTime class does not inherently support.
• Documentation and Tests: The README.md has been updated to reflect support for xsd:dateTime, and comprehensive unit tests have been added to validate the parsing, equality, and comparison logic of the XsdDateTime class.

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[gemini-code-assist]

Code Review

This pull request introduces support for the xsd:datetime data type. It adds a new XsdDateTime wrapper class to handle XSD's specific requirements for timezones, particularly 'floating' datetimes and preserving the original offset for round-tripping. A corresponding codec and unit tests are also included. The implementation is well-structured and the code is clear. I've left a couple of suggestions for minor improvements: one to simplify the code in the XsdDateTime class and another to enhance the test suite with cases for fractional seconds.

[gemini-code-assist]

The modulo operation % 1000 is redundant here. The DateTime.microsecond property is already documented to be in the range of 0 to 999. You can safely remove the modulo for cleaner code.

Suggested change

	final us = (dt.microsecond > 0)
	? (dt.microsecond % 1000).toString().padLeft(3, '0')
	: '';
	final us = (dt.microsecond > 0)
	? dt.microsecond.toString().padLeft(3, '0')
	: '';