Add Iterator.Buffered() method to mimic json.Decoder

[mhr3]

Summary

This PR adds a new Buffered() method to the Iterator that mimics the behavior of Go's standard library json.Decoder.Buffered().

Changes

✨ New Feature: Iterator.Buffered() method

• Added Iterator.Buffered() io.Reader - Returns a reader of the data remaining in the Iterator's buffer
• Standard library compatibility - Matches the behavior of json.Decoder.Buffered() from Go's standard library

🐛 Bug Fixes: Error Position Reporting

Fixed multiple instances of missing iter.unreadByte() calls, which allow using the Buffered more effectively.

These fixes ensure that when parsing errors occur, the iterator can return buffer including the problematic character rather than the character after it.

Use Cases

The Buffered() method enables:

• Incremental parsing: Process JSON incrementally while preserving unread data
• Parser chaining: Pass remaining data to other parsers or processors
• Debugging: Inspect remaining unparsed data for troubleshooting

Testing

• All new functionality is covered by comprehensive unit tests