Clean up extension from unnecessary PoC features

[Copilot]

This PR removes unnecessary features and over-engineered code from the VS Code extension to focus on core functionality and improve maintainability.

Changes Made

Removed Demo/PoC Features

• Hello World command: Removed the demo command from both package.json contributions and extension.ts
• Redundant activity bar view: Removed slingrExtensionView, keeping only the essential slingrExplorer

Simplified Drag & Drop System

• Completely removed complex field reordering functionality that used ts-morph for code manipulation
• Removed TreeDragAndDropController implementation from ExplorerProvider
• Eliminated handleDrag, handleDrop, and reorderFieldsAndGetText methods
• Removed associated MIME type handling and data transfer logic

Streamlined Cache System

• Removed over-engineered implicit field reference tracking (buildImplicitViewFieldReferences)
• Eliminated complex queue-based file processing in favor of direct change handling
• Removed detailed decorator argument parsing that wasn't being used
• Simplified reference tracking while maintaining essential navigation functionality
• Reduced complex method parameter parsing

Code Reduction

• ~1,300 lines removed across core files
• cache.ts: 60% reduction (850→344 lines)
• explorerProvider.ts: 65% reduction (570→209 lines)
• extension.ts: 18% reduction (60→49 lines)

Maintained Core Functionality

• Tree view explorer for Slingr data models
• Navigation to code definitions
• Model and field discovery from src/data/ directory
• File watching for cache updates
• All essential test coverage

Impact

The extension now focuses on its core purpose: providing a clean, maintainable tree view explorer for Slingr data models without the complexity of experimental features. This improves code readability, reduces compilation time, and makes future development easier.

Fixes #13.

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