Convert frontend code to typescript (#31559)

None of the frontend js/ts files was touched besides these two commands
(edit: no longer true, I touched one file in
61105d0618
because of a deprecation that was not showing before the rename).

`tsc` currently reports 778 errors, so I have disabled it in CI as
planned.

Everything appears to work fine.

web_src/js/features/file-fold.ts

@@ -0,0 +1,19 @@
+import {svg} from '../svg.ts';
+
+// Hides the file if newFold is true, and shows it otherwise. The actual hiding is performed using CSS.
+//
+// The fold arrow is the icon displayed on the upper left of the file box, especially intended for components having the 'fold-file' class.
+// The file content box is the box that should be hidden or shown, especially intended for components having the 'file-content' class.
+//
+export function setFileFolding(fileContentBox, foldArrow, newFold) {
+  foldArrow.innerHTML = svg(`octicon-chevron-${newFold ? 'right' : 'down'}`, 18);
+  fileContentBox.setAttribute('data-folded', newFold);
+  if (newFold && fileContentBox.getBoundingClientRect().top < 0) {
+    fileContentBox.scrollIntoView();
+  }
+}
+
+// Like `setFileFolding`, except that it automatically inverts the current file folding state.
+export function invertFileFolding(fileContentBox, foldArrow) {
+  setFileFolding(fileContentBox, foldArrow, fileContentBox.getAttribute('data-folded') !== 'true');
+}