Implement the ISO 8601-2:2019 negative duration extension (#37)

Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

README.markdown

@@ -65,6 +65,7 @@ expressed as JSON Schema definitions.
 | ISO | [ISO 4217:2015](https://www.iso.org/iso-4217-currency-codes.html) | Codes for the representation of currencies and funds |
 | ISO | [ISO 639:2023](https://www.iso.org/iso-639-language-code) | Codes for the representation of names of languages |
 | ISO | [ISO 8601-1:2019](https://www.iso.org/standard/70907.html) | Date and time — Representations for information interchange — Part 1: Basic rules |
+| ISO | [ISO 8601-2:2019](https://www.iso.org/standard/70908.html) | Date and time — Representations for information interchange — Part 2: Extensions |
 | ISO | [ISO 80000-1:2022](https://www.iso.org/standard/76921.html) | Quantities and units — Part 1: General |
 | ISO/IEC | [ISO/IEC 2382:2015](https://www.iso.org/standard/63598.html) | Information technology — Vocabulary |
 | ISO/IEC | [ISO/IEC 9899:2024](https://www.iso.org/standard/82075.html) | Programming languages — C |