[meta] Add comprehensive .github/copilot-instructions.md for better AI code assistance

This comprehensive guide helps GitHub Copilot better understand and navigate the nvm codebase by providing:

- Complete architecture explanation of nvm's POSIX-compliant design across multiple shells
- Detailed mapping of top-level commands to internal functions (install, use, ls, alias, etc.)
- Comprehensive testing infrastructure guide using urchin framework
- Shell environment setup for bash, zsh, dash, sh, and ksh
- CI environment details for GitHub Actions and Travis CI
- shellcheck setup and usage guidelines
- Development best practices for POSIX compliance and multi-shell compatibility
- Windows support via WSL2, Cygwin, and Git Bash
- Complete internal function categorization and debugging techniques

The documentation covers all aspects of the codebase to enable more accurate AI assistance when working with nvm's complex shell script architecture.

Author: Copilot