Add JSONKit to Command-line tools

[vesper-astrena]

Added JSONKit to the Command-line tools section.

JSONKit is a zero-dependency Python CLI tool for working with JSON:

• Format/pretty-print with syntax highlighting
• Validate with error location
• Query using dot-notation (data.users[0].name)
• Diff two JSON files
• Flatten/unflatten nested structures
• JSON → CSV conversion
• Structure stats (depth, keys, types)
• Pipe/stdin support

Single Python file, no pip install needed. Python 3.10+.

Happy to adjust the entry if needed!