Add --screenshot functionality to mdx web command

[copilot-developer-agent-robch]

The --screenshot [FILE] option has been added to the mdx web command to automatically capture a screenshot of each web page after loading. The screenshot file path uses the same template functionality as other outputs like --save-page-output, where:

• If no file path is specified, the default template "{filePath}/{fileBase}-output.png" is used
• Template variables like {filePath} and {fileBase} are supported to generate unique filenames
• Screenshots are taken in headless browser mode by default unless --interactive is used
• Full page screenshots are captured to include all content, not just the viewport
• Screenshot paths are included in the markdown output for reference

Example usage:

# Save screenshot with default filename template 
mdx web get https://example.com --screenshot

# Use custom screenshot filename template
mdx web get https://example.com --screenshot "{filePath}/{fileBase}-fullpage.png"

# Screenshot multiple pages, saving content and screenshots
mdx web get https://example.com https://another.com --save-page-output "{filePath}/{fileBase}-content.md" --screenshot "{filePath}/{fileBase}-screenshot.png"

The required changes included:

• Adding Screenshot property to WebCommand base class
• Adding screenshot command line option parsing
• Extending PlaywrightHelpers to support screenshot capture
• Updating help documentation with new parameter

Fixes #5.