Add GFM alerts extension

[ia3andy]

Note

I have used Claude to code most of it, I am currently reviewing the generated code which seems to be overall valid. I've requested to follow the pattern of existing extensions (and compare to other implementations of this feature).

Summary

• Adds new extension module for GitHub Flavored Markdown alerts
• Implements parser for alert blockquote syntax (> [!NOTE], > [!WARNING], etc.)
• Includes HTML, Markdown, and text content renderers
• Supports all five standard GFM alert types: NOTE, TIP, IMPORTANT, WARNING, CAUTION
• Allows custom alert types via builder API
• Includes comprehensive test coverage and documentation

Closes #327

[ia3andy]

I've been through the code which seems good to me so I can say I am taking responsibilities for it like my own. Still, another set of eyes would be good :)