include mapping spec language definition from mapping table research

[TRThurman]

@ronaldtse , @stuartgalt, @considiner-boeing , Let's use this schema for a starting point for the planned updates:
https://github.com/TRThurman/iso_10303_map_editor/blob/main/research/mapping-schema-v3.yaml.

It is already used in the mapping spec editor vs code plugin.

[TRThurman]

The mapping specification schema has been drafted and validated against 588 mapping.yaml files in wg12-step (zero errors):

• Add mapping specification schema (ELF 5006 Annex B) schemas#2 — adds mapping/v1/mapping_specification.yaml (JSON Schema draft-07) based on ELF 5006 Annex B (merged in Add EXPRESS links and YAML serialization annex #10)
• Schema covers: entity mappings, attribute assertions, reference paths, refpath_extend/extended_select, alternative mappings, subtype constraints
• Derived from mapping-schema-v3.yaml with updates for post-migration format (EXPRESS links, source removed, aimelt flattened)

The v3 schema referenced in this issue has been updated to v4 at https://github.com/TRThurman/iso_10303_map_editor/blob/main/research/mapping-schema-v4.yaml