Support Ctrl+scroll wheel zoom for text size in the Markdown WYSIWYG editor

The new built-in Markdown WYSIWYG editor (tracked in
#296639) doesn't currently support enlarging text via mouse scroll. The standard text editor supports Ctrl + scroll wheel to zoom font size (editor.mouseWheelZoom), and the Markdown preview's Mermaid diagrams support Alt+scroll zoom — but the new WYSIWYG editing surface has no equivalent for its rendered/edited text.

**Describe the solution you'd like**
Add support for Ctrl + mouse wheel scroll to increase/decrease the font size within the WYSIWYG editor view, consistent with the zoom behavior already available in the standard text editor.

**Describe alternatives you've considered**

VS Code's global zoom (Ctrl +/Ctrl -) — resizes the entire window/UI, not just the editing surface.
Manually increasing font size via settings — works but is slow and not a per-document adjustment.

**Additional context**
My eyesight has been getting worse, and quick scroll-to-zoom is a much faster and more natural way to adjust text size on the fly than digging through settings each time. Since this WYSIWYG editor is brand new, it would be a great time to bake this in early. Referencing
#296639 for context on the editor this applies to.