refactor!: restructured api to use video_player controller

[OutdatedGuy]

fixes #41
closes #42
closes #43

v4.0.0: Complete API Restructuring & Major Improvements

🚨 BREAKING CHANGES:

• Complete API overhaul: Replace controller + widget pattern with unified class-based API
• Class consolidation: Remove CachedVideoPlayerPlusController, CachedVideoPlayerPlusValue, and CachedVideoPlayerPlus widget
• Import changes: Now requires import 'package:video_player/video_player.dart' for VideoPlayer widget
• Method renames: removeCurrentFileFromCache() → removeFromCache()
• Parameter type changes: removeFileFromCache(String) → removeFileFromCache(Uri)
• Constructor changes: All constructors moved from controller to main class

✨ Features:

• Unified API: Single CachedVideoPlayerPlus class handles both caching logic and controller management
• Enhanced documentation: Comprehensive inline documentation following Dart conventions with detailed examples
• Debug optimization: All debug prints wrapped in kDebugMode guards for better performance
• Complete migration guide: Detailed step-by-step migration documentation with examples
• Modernized example: Updated example app showcasing new API patterns

🚀 Improvements:

• Massive codebase reduction: Simplified architecture while maintaining full functionality
• Better separation of concerns: Clear distinction between caching logic and video playback
• Enhanced error handling: Improved validation and user-friendly error messages
• Future-proof design: Built on modern Flutter patterns and best practices
• Performance optimizations: Streamlined initialization and caching workflows

[OutdatedGuy]

LGTM!