Make markdown library .set() method optional

Author: danburzo

src/Engines/Markdown.js

@@ -18,7 +18,7 @@ class Markdown extends TemplateEngine {
 
     // Overrides a highlighter set in `markdownOptions`
     // This is separate so devs can pass in a new mdLib and still use the official eleventy plugin for markdown highlighting
-    if (this.config.markdownHighlighter) {
+    if (this.config.markdownHighlighter && typeof this.mdLib.set === "function") {
       this.mdLib.set({
         highlight: this.config.markdownHighlighter,
       });