Add append operation for efficient text streaming

[jeonsworld]

Summary

This PR adds a new append operation to python-json-patch for efficient text streaming in chat applications. When streaming text character-by-character or word-by-word, the standard replace operation requires sending the entire string with each update. The new append operation only sends the incremental text, significantly reducing payload size and improving performance.
This method is used by official ChatGPT and is an implementation optimized for chat.

Key Features

• New append operation: Appends text to existing string values
• Optimized notation support: Reduces payload size for consecutive operations
• Smart patch generation: make_patch() automatically detects append scenarios
• Full backward compatibility: Maintains RFC 6902 compliance

Implementation Details

1. AppendOperation class: New operation type that appends text to strings at specified JSON paths

    {"op": "append", "path": "/message/content", "value": "Hello"}

2. Optimized notation for streaming:

    - Standard: {"op": "append", "path": "/message", "value": "text"}
    - Short: {"p": "/message", "o": "append", "v": "text"}
    - optimized: {"v": "text"} (reuses previous path/operation)

3. patch generation: When using make_patch(), the library now detects when a string change is an append
   operation:

  src = {"message": "Hello"}
  dst = {"message": "Hello World"}
  patch = jsonpatch.make_patch(src, dst)
  # Generates: [{"op": "append", "path": "/message", "value": " World"}]
  # Instead of: [{"op": "replace", "path": "/message", "value": "Hello World"}]

Performance

The demo script shows append operations are ~2.8x faster than replace operations for streaming scenarios.