[Benchmark] Add support for RefCOCO-M benchmark

[rshube]

Summary

This PR adds support for the RefCOCO-M benchmark in VLMEvalKit.

Changes

• adds a dedicated RefCOCOMDataset
• registers RefCOCO-M in the dataset registry
• keeps the benchmark separate instead of treating it as an alias of RefCOCO

Dataset format

Expected TSV columns:

• index
• image
• question
• answer
• bbox_x1
• bbox_y1
• bbox_x2
• bbox_y2
• width
• height
• category

Evaluation

This benchmark reuses the existing RefCOCO-style grounding evaluation path.

Notes

• this PR only adds benchmark support code
• hosted DATASET_URL / DATASET_MD5 can be filled once the final TSV hosting location is settled (sending email to maintainers)