[Feature request]: add contrast provider

[ihardlight]

I would like to be able to get not only theme from provider but user contrast preferences as well
I have developed a new library called “next-contrast,” which functions similarly to “next-themes” but with a focus on contrast.

📌 Summary

This PR introduces a new ContrastProvider and useContrast API to improve accessibility by enabling explicit support for the prefers-contrast media query. These additions make it possible to handle high-contrast themes in a declarative and ergonomic way within the existing next-themes ecosystem.

🧩 What’s Included
• ✅ ContrastProvider: context provider that detects and responds to user’s contrast preferences.
• ✅ useContrast hook: allows access to the current contrast mode and preference in a fully typed and reactive way.
• ✅ SSR-safe hydration and fallback handling for contrast preference detection.
• ✅ E2E test coverage for both system-based and user-defined contrast modes.
• ✅ Documentation updates in the README, including usage examples and configuration options.

🎯 Motivation

Many operating systems support user preferences for high or low contrast, which is essential for users with vision impairments or accessibility needs. However, there has been no built-in way to integrate this into next-themes. By adding a ContrastProvider, developers can now:
• Respect and respond to system contrast settings.
• Explicitly define and persist contrast themes
• Build UIs that adapt to both theme and contrast preferences.

This lays the groundwork for building more accessible, flexible, and user-centered Next.js applications.

[ihardlight]

@pacocoursey Hi! What do you think about supporting high contrast themes?