fix: index.css/js to styles/script (#44356)

* fix: replace index with script/styles as needed

* fix: remove redundant fileKey

It's overwritten by createPoly, so the parser does not need to create it

* fix: curriculum test suite

* Update client/src/templates/Challenges/classic/MultifileEditor.js

Co-authored-by: Shaun Hamilton <shauhami020@gmail.com>

Co-authored-by: Shaun Hamilton <shauhami020@gmail.com>

client/src/templates/Challenges/rechallenge/transformers.js

@@ -227,11 +227,11 @@ const transformIncludes = async function (fileP) {
     div.querySelector('script[src="./script.js"]');
   const importedFiles = [];
   if (link) {
-    importedFiles.push('index.css');
+    importedFiles.push('styles.css');
     link.remove();
   }
   if (script) {
-    importedFiles.push('index.js');
+    importedFiles.push('script.js');
     script.remove();
   }