Adds a FenceRendererExtension for language to the extension list.
public static void AddFenceRenderer(this IList<IMarkViewExtension> extensions, string language, FenceRendererFactory factory)
// Global (App.axaml.cs)
MarkdownViewerDefaults.Extensions.AddFenceRenderer("chart", (code, info) => new MyChart(code));
// Per-instance
viewer.Extensions.AddFenceRenderer("chart", (code, info) => new MyChart(code));