Adds TextMateExtension to the extension list.
public static void AddTextMateHighlighting(this IList<IMarkViewExtension> extensions, ThemeName darkTheme = ThemeName.DarkPlus, ThemeName lightTheme = ThemeName.LightPlus)
// Global (App.axaml.cs)
MarkdownViewerDefaults.Extensions.AddTextMateHighlighting();
// Per-instance
viewer.Extensions.AddTextMateHighlighting();