feat: add new ranking tasks for melo

[federetyk]

Addresses #30

Description

This PR introduces the MELO Benchmark (Multilingual Entity Linking of Occupations) as a new ranking task for job title normalization into ESCO. MELO provides 42 evaluation datasets spanning 21 languages, built from crosswalks between national occupation taxonomies and ESCO published by official labor-related organizations across EU member states.

Additionally, we include MELS (Multilingual Entity Linking of Skills), a sibling benchmark following the same methodology but targeting skill normalization into ESCO Skills rather than occupations. MELS currently covers 5 languages with 8 datasets, providing complementary evaluation coverage for the skill normalization task group.

This PR is built on top of #34, which introduces a refactor with the generalized dataset indexing infrastructure required for this implementation. As such, this PR is contingent on #34 being merged. If the maintainers prefer a different approach for the refactor, I would be happy to adapt the implementation accordingly.

Changes:

• Add MELORanking task class with 42 datasets across 21 languages for job normalization
• Add MELSRanking task class with 8 datasets across 5 languages for skill normalization
• Extend RankingDataset constructor to support allow_duplicate_targets parameter (required by MELO)
• Add unit tests for dataset ID filtering logic with various language combinations
• Add defensive check in e2e test to skip tasks with no datasets for the requested language set
• Update README with new task entries

Checklist

• Added new tests for new functionality
• Tested locally with example tasks
• Code follows project style guidelines
• Documentation updated
• No new warnings introduced