Add compare selection with clipboard functionality

[Copilot]

This PR implements the feature request to compare text selection with clipboard content, extending the existing diff & merge functionality with two new commands:

🎯 New Features

1. Compare Selection with Clipboard

• Command: diffMerge.compareSelectionWithClipboard
• Usage: Select text in any editor, then use the command to compare the selection with clipboard content
• Access: Available via editor context menu, editor title context menu, and command palette (when text is selected)

2. Compare Current vs Previous Clipboard

• Command: diffMerge.compareClipboardWithPrevious
• Usage: Compares current clipboard content with previously saved clipboard content
• Behavior: First run saves current clipboard as "previous", subsequent runs compare current vs saved
• Access: Available via editor title context menu and command palette

🔧 Implementation Details

Both features follow the existing code patterns:

• Use the same showDiff function as other comparison commands
• Provide appropriate user feedback (e.g., when no text is selected)
• Integrate seamlessly with existing menu structures
• Follow the extension's naming conventions and UI patterns

Example usage:

1. Select some text in an editor
2. Right-click → "[Diff & Merge] Compare selection with Clipboard"
3. View diff between your selection and clipboard content

The implementation is minimal and surgical, adding only the necessary functionality without modifying existing features.

Fixes #119.

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