Add YMODEM protocol support

[renbingcheng]

Description

This PR adds YMODEM protocol support to the existing XMODEM implementation. YMODEM is built on top of XMODEM with additional file metadata capabilities.

Key Features Added

• Full YMODEM protocol implementation (based on XMODEM-1K)
• File header transmission (filename + size metadata)
• Batch file transfer capability
• Backward compatible with existing XMODEM usage

Implementation Details

• Extended XMODEM class with ymodem mode support
• Added file info packet handling (_send_file_info method)
• Maintained all existing XMODEM functionality
• Followed the project's existing code style and patterns

Testing

The implementation has been tested with:

• Single file transfers
• Various file sizes (small to large)
• CRC checksum validation

Documentation

Updated README and docstrings to reflect the new YMODEM support.

Backward Compatibility

Existing XMODEM functionality remains unchanged. The new YMODEM support is opt-in via the mode='ymodem' parameter.