Add plugin-tee and introduce breaking WIT changes for enhanced plugin capabilities

[topheman]

This PR introduces the new plugin-tee plugin and makes necessary breaking changes to the WebAssembly Interface Types (WIT) contracts to enable enhanced plugin functionality.

🆕 New Features

plugin-tee

A new Rust-based plugin that implements the Unix tee command functionality:

• File creation: Creates new files with content from the previous command output ($0)
• File overwriting: Replaces existing file content
• Append mode: Supports -a or --append flags to append content to existing files
• Error handling: Provides detailed error messages for permission issues and file operations
• Integration: Works seamlessly with other plugins like echo and cat in the REPL workflow

The plugin demonstrates the new plugin API capabilities by accessing REPL variables through the enhanced host interface.

⚠️ Breaking Changes

1. WIT Contract Changes (plugin-api.wit)

• Added: host-state-plugin interface with get-repl-var function
• Purpose: Allows plugins to access specific REPL variables by name (e.g., $0 for previous command output)
• Impact: All existing plugins must be recompiled against the new WIT contracts

2. WIT Contract Changes (host-api.wit)

• Removed: set-repl-var function from host-state interface
• Reason: Functionality moved to plugin-specific interface for better separation of concerns
• Impact: REPL logic components must be updated to use the new interface structure

🔧 Technical Implementation

• Plugin Host: Enhanced with thread-safe REPL variable sharing using Arc<Mutex<T>>
• WASM Integration: New bindings generated for all plugins to support the updated WIT contracts
• Testing: Comprehensive test coverage for plugin-tee including file creation, overwriting, and append scenarios
• Permissions: Plugin requires appropriate filesystem permissions (--allow-all for full functionality)

📋 Migration Guide

For Plugin Developers:

1. Recompile plugins against the new WIT contracts
2. Update any code that previously used set-repl-var to use the new plugin-specific interfaces
3. Consider using the new get-repl-var function for accessing REPL state

For REPL Logic Components:

1. Update to use the new WIT contract structure
2. Ensure compatibility with the removed set-repl-var function

🧪 Testing

• ✅ plugin-tee functionality (new file, overwrite, append)
• ✅ Integration with existing plugins (echo, cat)
• ✅ Error handling and permission management
• ✅ WIT contract compatibility across all components

📚 Documentation

• Added comprehensive README for plugin-tee
• Updated WIT interface documentation
• Enhanced test coverage demonstrating plugin capabilities

🌐 Web Version Note

The web version of plugin-tee is not part of this PR, it will be added in another one.

This release represents a significant step forward in plugin capabilities while maintaining the REPL's core functionality and security model.