Add lightweight GitHub alerts and accessible task-list rendering

[trsdn]

Goal

Support common GitHub Markdown conventions in both editions without another parser dependency.

Requirements

• Recognize NOTE, TIP, IMPORTANT, WARNING, and CAUTION blockquote alerts.
• Render alert structure using constrained DOM construction and existing sanitizer/CSP boundaries.
• Style alerts with palette variables and non-color-only labels/icons.
• Render GFM task-list checkboxes accessibly while keeping Markdown source authoritative; preview interaction must never silently mutate text.
• Preserve nested lists, source order, live preview, printing, and plain fallback for malformed syntax.
• Add no remote assets, icon font, or third-party JavaScript.

[trsdn]

Implemented and released in MDViewer+ v2.0.0.