feat: enable reset in multifile editor (#43617)

* feat: dispatch resetChallenge action

* fix: copy challengeFiles instead of in-place sort

* fix: handle null updateFile payloads in redux

* refactor: reorganise region initialization

* refactor: pull code into editorDidMount

Then we can rely on the presence of the editor and monaco and don't have
litter the code with null checks.

* refactor: use better name for editable region init

* refactor: remove unused decoration

* refactor: rename forbidden region init functions

* fix: keep all challengeFiles when resetting

* refactor: remove unused decoration class

* fix: reinitialize editor on reset

* fix: stop adding multiple scroll listeners

Since the challengeFile update on each keystroke extra (unnecessary)
adding of listeners slowed the editor to a crawl.

* fix: only scroll to editor on mount

Rather than on any edit.

* refactor: remove logs and comments

* fix: rename toSortedArray and fix broken test

* fix: check for null not falsy in updateFile

* fix: only update project features when project

* fix: only reset if editor contents have changed

* feat: focus on editor after reset

utils/sort-challengefiles.js

@@ -0,0 +1,17 @@
+exports.sortChallengeFiles = function sortChallengeFiles(challengeFiles) {
+  const xs = challengeFiles.slice();
+  // TODO: refactor this to use an ext array ['html', 'js', 'css'] and loop over
+  // that.
+  xs.sort((a, b) => {
+    if (a.ext === 'html') return -1;
+    if (b.ext === 'html') return 1;
+    if (a.ext === 'css') return -1;
+    if (b.ext === 'css') return 1;
+    if (a.ext === 'jsx') return -1;
+    if (b.ext === 'jsx') return 1;
+    if (a.ext === 'js') return -1;
+    if (b.ext === 'js') return 1;
+    return 0;
+  });
+  return xs;
+};