Add comprehensive demo showcasing all Rainbow features

- Rewrite Playground/main.swift with systematic feature demonstrations
- Cover basic colors, light colors, background colors, text styles
- Add 8-bit color palette, 24-bit RGB, hex colors, HSL colors examples
- Include builder pattern and batch API demonstrations
- Add practical examples: log levels, progress bar, status messages
- Preserve Chinese poetry examples for cultural showcase
- Update README with instructions to run the demo

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: onevdog