Registers MermaidBlockRenderer so it intercepts ```mermaid fences
before CodeBlockRenderer. Scans for the first renderer that would accept a
Markdig.Syntax.FencedCodeBlock and inserts just before it, rather than a fixed
Insert(0, ...) — that way a narrower, more specific fenced-block renderer
(e.g. MarkView.Avalonia.Math's MathBlockRenderer, which only accepts its own
MathBlock subtype) registered before or after this one is never displaced from the
front of the list, regardless of registration order.
public MermaidExtension(MermaidStyle style = MermaidStyle.Quiet)
The visual style diagrams are painted in.