refactor: files{} -> challengeFiles[], and key -> fileKey (#43023)

* fix(client): fix client

* fix propType and add comment

* revert user.json prettification

* slight type refactor and payload correction

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

* update ChallengeFile type imports

* add cypress test for code-storage

* update test and storage epic

* fix Shaun's tired brain's logic

* refactor with suggestions

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

* update codeReset

* increate cypress timeout because firefox is slow

* remove unused import to make linter happy

* use focus on editor

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

* use more specific seletor for cypress editor test

* account for silly null challengeFiles

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

utils/__fixtures__/challenges.js

@@ -1,50 +1,50 @@
-exports.challengeFiles = {
-  indexcss: {
+exports.challengeFiles = [
+  {
     contents: 'some css',
     error: null,
     ext: 'css',
     head: '',
     history: ['index.css'],
-    key: 'indexcss',
+    fileKey: 'indexcss',
     name: 'index',
     path: 'index.css',
     seed: 'some css',
     tail: ''
   },
-  indexhtml: {
+  {
     contents: 'some html',
     error: null,
     ext: 'html',
     head: '',
     history: ['index.html'],
-    key: 'indexhtml',
+    fileKey: 'indexhtml',
     name: 'index',
     path: 'index.html',
     seed: 'some html',
     tail: ''
   },
-  indexjs: {
+  {
     contents: 'some js',
     error: null,
     ext: 'js',
     head: '',
     history: ['index.js'],
-    key: 'indexjs',
+    fileKey: 'indexjs',
     name: 'index',
     path: 'index.js',
     seed: 'some js',
     tail: ''
   },
-  indexjsx: {
+  {
     contents: 'some jsx',
     error: null,
     ext: 'jsx',
     head: '',
     history: ['index.jsx'],
-    key: 'indexjsx',
+    fileKey: 'indexjsx',
     name: 'index',
     path: 'index.jsx',
     seed: 'some jsx',
     tail: ''
   }
-};
+];