Add extended synonyms computation

[alexgarel]

What

• We want to be able to add extended synonyms.
• It's not specified in doc/references/taxonomy-parser.md, but the specification is the perl code ! 🙈 (if possible add also a specification)
• You might either implement them using Neo4J capabilities, or simply with iterator over existing nodes like in perl code.
• Extended synonyms should be added in a specific field on the neo4j entry (no need to repeat existing entries), tags_extended_LC (LC is to be replaced by language code) and their normalized form: tags_ids_extended_LC.
• Augment the parsing example to include extended synonyms.
• See also Add a test on taxonomy builder openfoodfacts-server#7394
• When you finish this ticket, add a new issue, to display them in the taxonomy-editor application :-)

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