feat: Add CLI interface and PDF export functionality

[Aaryyan777]

This PR introduces two new features to mdify:

1. New CLI Interface (src/cli.ts)

I built a Command Line Interface to allow using mdify directly from the terminal.

• Interactive Mode: Prompts the user for a URL if none is provided.
• Batch Processing: Support for converting multiple URLs via a file (--batch).
• Clipboard Support: Option to auto-copy result to clipboard (--copy).
• UI: Includes spinners, colorized outputs, and a custom ASCII logo.

2. PDF Export Support

I integrated html2pdf.js into the toolbar to allow users to download articles as PDFs.

• Quality: configured for 4x scale rendering for sharp text.
• Styling: Added CSS rules to prevent awkward page breaks inside code blocks and images.

3. Testing & CI

• Added Vitest configuration and unit tests for the conversion logic.

[vercel]

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