Move subset-dataset.js to main examples directory

- Move subset-dataset.js from opendap-browser/ to examples/ directory
- Places the coordinate selection example alongside other main examples
- Makes it easier to find and run the XArray-style API demonstration

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: mpiannucci