Add --wait-for UI element capabilities to mdx web command

[copilot-developer-agent-robch]

Added support for waiting on specific UI elements before proceeding with web operations. This is particularly useful when dealing with dynamic content that may take time to load.

Key changes:

• Added WaitForSelectors property to WebCommand class to track selectors to wait for
• Added --wait-for option parsing to accept one or more CSS selectors
• Updated PlaywrightHelpers.cs to wait for specified selectors with timeout
• Added documentation and examples in help files

Example usage:

# Wait for main content and a dynamic widget to load
mdx web get https://example.com --wait-for "#main-content" ".dynamic-widget"

# Wait for a specific element in dynamic form
mdx web get https://example.com/form --wait-for ".form-loaded" 

The command will wait up to 30 seconds for each specified selector to appear before continuing. If an element doesn't appear within the timeout period, an error is thrown with details about which selector failed to load.

This feature makes web scraping more reliable when dealing with websites that load content dynamically or use JavaScript frameworks.

Fixes #7.