Modernize code and dependencies for React 18 and 19

[jessetan]

This PR upgrades react-headroom to be compatible with React 19 while maintaining backward compatibility with React 18. It also modernizes the entire dependency ecosystem and toolchain.

This was co-written with AI, but guided and tested with love and care by a real human.

As far as I can see, this is functionally equivalent to the current package, I have not updated the package version nor the README. The doc site was removed as it would be another migration.

I hope this PR will help others, even if it never gets merged.

Key Changes

React 19 Compatibility

• Fixed defaultProps deprecation warning
• Updated peerDependencies to support React ^18 || ^19
• Removed PropTypes (no longer needed in modern React)

Dependency Modernization

• Babel 6 → 7 ecosystem upgrade with modern presets and plugins
• ESLint 3 → 9 with flat config system replacing legacy .eslintrc
• Mocha and Chai updated to latest versions with ESM compatibility
• Modern pre-commit hooks using @fastify/pre-commit
• Replaced raf polyfill with native requestAnimationFrame (supported by modern browsers)

Breaking Changes

• Minimum Node.js version may be higher due to modern dependencies
• Removed support for React <18

Migration

Users on React 18+ can upgrade directly. For React <18, stay on the previous version.

The upgrade maintains all existing APIs and functionality while eliminating deprecation warnings and providing a modern development experience that is compatible with latest React.