Enable ESLint rule unicorn/no-array-for-each, auto-fix all violations, manually address new issues for ~4% time reduction measured via profile-fixture.mjs on Apple Silicon M1.

lib/md039.js

@@ -18,7 +18,7 @@ module.exports = {
       let inLink = false;
       let linkText = "";
       let lineIndex = 0;
-      children.forEach((child) => {
+      for (const child of children) {
         const { content, markup, type } = child;
         if (type === "link_open") {
           inLink = true;
@@ -61,7 +61,7 @@ module.exports = {
             `${markup}${content}${markup}` :
             (content || markup);
         }
-      });
+      }
     });
   }
 };