Add Format-Specific Code Highlighting Stylesnew change

[LuHC409]

Add Format-Specific Code Highlighting Styles

Description

Added the ability to specify different code highlighting styles for different output formats (pdf, html, png, jpeg). For example, users can now use light themes for PDF files (for printing) and dark themes for HTML files.

Changes

1. Modified the readStyles function in extension.js to support format-specific code highlighting styles
2. Updated the markdown-pdf.highlightStyle configuration in package.json to support object-type configuration

Configuration Example

Users can now configure different highlighting styles for different formats:

{
  "markdown-pdf.highlightStyle": {
    "pdf": "github.css",     // Light theme for PDF files
    "html": "monokai.css",   // Dark theme for HTML files
    "default": "github.css"  // Default style for other formats
  }
}

Or continue using a single highlighting style:

{
  "markdown-pdf.highlightStyle": "github.css"
}

Backward Compatibility

• Maintains backward compatibility with single string configuration
• Provides default style fallback mechanism
• All available styles can be previewed at highlight.js demo

Testing

• Tested code highlighting for PDF output format
• Tested code highlighting for HTML output format
• Tested backward compatibility with default configuration
• Tested switching between different highlighting styles

Related Issue

Closes #398

Additional Notes

• Complete list of available highlighting styles can be found at highlight.js/src/styles
• Style previews are available at highlight.js demo