Add ReerKit

[vnixx]

• Project Name: ReerKit
• Project URL: https://github.com/reers/ReerKit
• Project Description: Powerful Swift foundation library of extensions and providing utility functions to supercharge your iOS/macOS/Linux development workflow.
• Why it should be added to awesome-swift: ReerKit is a comprehensive foundation library that enhances Swift development with over 500+ carefully crafted extension methods and utility functions.
• At least 15 stars (GitHub project)
• Support Swift 5
• Updated contents.json instead of README
• Lib is fully open sourced, written in Swift and not a wrapper over compiled lib
• Description does not say "written in Swift" or variant 🤓