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>

tools/challenge-parser/parser/plugins/__snapshots__/add-seed.test.js.snap

@@ -10,7 +10,6 @@ Object {
 </html>",
       "editableRegionBoundaries": Array [],
       "ext": "html",
-      "fileKey": "indexhtml",
       "head": "",
       "id": "",
       "name": "index",
@@ -22,20 +21,18 @@ Object {
 }",
       "editableRegionBoundaries": Array [],
       "ext": "css",
-      "fileKey": "indexcss",
       "head": "",
       "id": "",
-      "name": "index",
+      "name": "styles",
       "tail": "",
     },
     Object {
       "contents": "var x = 'y';",
       "editableRegionBoundaries": Array [],
       "ext": "js",
-      "fileKey": "indexjs",
       "head": "",
       "id": "",
-      "name": "index",
+      "name": "script",
       "tail": "",
     },
   ],