fix: throw error when async function is not marked async (#91)

spec/index.test.js

@@ -162,4 +162,16 @@ no need to escape chars
 </code></pre>"
 `);
   });
+
+  test('async highlight without async option', async() => {
+    marked.use(markedHighlight({
+      highlight(code, lang) {
+        return new Promise((resolve, reject) => {
+          resolve(code);
+        });
+      }
+    }));
+
+    expect(() => marked(markdown)).toThrow(/set the async option to true/i);
+  });
 });

src/index.js

@@ -27,6 +27,9 @@ export function markedHighlight(options) {
       }
 
       const code = options.highlight(token.text, lang);
+      if (code instanceof Promise) {
+        throw new Error('markedHighlight is not set to async but the highlight function is async. Set the async option to true on markedHighlight to await the async highlight function.');
+      }
       updateToken(token)(code);
     },
     renderer: {

types_test/index.test-d.ts

@@ -4,6 +4,9 @@ import {expectError} from 'tsd';
 // Single function argument
 markedHighlight((code: string, language: string) => code+language);
 
+// Invalid asynchronous function argument - missing async: true
+expectError(markedHighlight(async (code: string, language: string) => code+language));
+
 // Minimal synchronous options argument
 markedHighlight({
   highlight(code: string, language: string) {