Add special case to webpack config for step1-04 docs

webpack.config.js

@@ -40,6 +40,9 @@ fs.readdirSync('./').filter(step => {
 
   if (!isEntryPoint && isValidLessonFolder(step)) {
     nonWebpackedEntries.push(step);
+  } else if (step === 'step1-04') {
+    // special case: this folder's `final` has code, but its `demo` doesn't
+    nonWebpackedEntries.push('step1-04/demo');
   }
 });