Add support for Abbreviations and Mnemonics

[daviddeelo]

Requesting to add support for Abbreviated terms and Mnemonics as a subsections to the base Clause 3 construct to be in alignment with ISO Directives Part 2 Clause 17 "Symbols and abbreviated terms"

This also includes support for issue #8

See ISO/IEC 23090-31:2025 for reference implementation:

[SteveLLamb]

Discussed 2026-07-17, guidelines to discuss for using this sub-section:

1. Abbr is the most common used format (i.e. JSON)
2. Used more than once in the document (and not inline of the prose, i.e "Key Delivery Message (KDM)")
3. Common enough that it would not need to be defined in the main Terms clause (and have the Abbr defined there)
4. CAVEAT - doesn't restrict being both ABBR in Terms (and) being listing in Abbr Sub-section