Add custom fonts support for Unicode PDFs

[kunal-595]

Closes #455

This PR adds support for custom Unicode fonts in PDF generation, allowing users to render Unicode characters (Arabic, Chinese, Japanese, etc.) by providing their own font files.

Changes

• Modified PdfTransformerBase.js to accept an optional fonts parameter in pdfMakeToPdfStreamWithCallback() and pdfMakeToPdfStream() methods
• Updated PdfTransformer.js to accept and pass through the fonts parameter in pdfMakeToPdfBuffer() method
• Exported defaultFonts from packages/markdown-pdf/index.js to allow users to extend the default Liberation fonts
• Added documentation in packages/markdown-pdf/README.md with usage examples showing how to use custom Unicode fonts
• Maintained full backward compatibility - existing code works unchanged

Flags

• Breaking Changes: None - the fonts parameter is optional and defaults to existing Liberation fonts
• Testing: Existing unit tests should pass unchanged; new functionality allows Unicode rendering when custom fonts are provided
• Dependencies: No new dependencies added

Screenshots or Video

N/A - API enhancement for font support

Related Issues

• Issue #455
• Pull request #634

Author Checklist

• [x] Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• [x] Vital features and changes captured in unit and/or integration tests
• [x] Commits messages follow AP format
• [x] Extend the documentation, if necessary