Add zx-compatible API with superior built-in commands and streaming

[konard]

Goal: Beat Google's zx by providing their convenience plus unique advantages

zx Advantages:

• Google backing and popularity growth
• Modern JavaScript features
• Good documentation

Our Advantages to Leverage:

• ✅ Built-in commands (18 vs 0) - no system dependencies
• ✅ Virtual commands engine (revolutionary vs none)
• ✅ Real-time streaming (vs buffered only)
• ✅ Smaller bundle (~20KB vs ~400KB+)
• ✅ EventEmitter pattern (vs none)
• ✅ Async iteration (vs none)
• ✅ Public domain license (vs Apache 2.0)
• ✅ Better signal handling

Implementation:

• Add $.zx compatibility mode
• Create #!/usr/bin/env command-stream shebang support
• Add global imports like zx ($, cd, fs, etc.)
• Document migration from zx scripts
• Show built-in commands advantage (no system deps)
• Bundle size comparison (20KB vs 400KB+)
• Streaming examples that zx can't do

Key Differentiators:

• Cross-platform scripts (built-in commands)
• Real-time processing (streaming)
• Custom command extensions (virtual commands)
• Smaller footprint
• No system dependencies

Success Metrics:

• Complete zx script compatibility
• Built-in commands showcase
• Bundle size advantages documented