[Benchmark] Add support for PixmoPoints benchmark

[rshube]

Summary

This PR adds support for the PixmoPoints benchmark in VLMEvalKit.

Changes

• adds a dedicated PixmoPointsDataset
• registers PixmoPoints in the dataset registry
• adds benchmark-specific point localization evaluation
• uses the repo-standard point_2d JSON prediction format

Dataset format

Expected TSV columns:

• index
• image
• question
• answer
• width
• height
• category

Evaluation

The evaluator performs point matching against normalized ground-truth points and reports:

• precision
• recall
• f1

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)