docs: Add comprehensive ROADMAP for future development

Document potential features and enhancements including:
- Additional pattern formats (Docker, Mercurial, npm, rsync)
- Performance enhancements (alternative regex backends, caching)
- Experimental Rust native backend exploration
- Pattern validation and linting tools
- Advanced filtering (case-insensitive, attribute-based)
- API enhancements (streaming, introspection, composition)
- Testing and tooling improvements

All items are organized by category with clear implementation
approaches, use cases, and considerations. No timelines specified.

Rust backend section emphasizes learning goals and exploratory
nature, with pure Ruby remaining the primary implementation.

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

Author: highb