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/autofocus-end.js

@@ -1,6 +0,0 @@
-export function initAutoFocusEnd() {
-  for (const el of document.querySelectorAll('.js-autofocus-end')) {
-    el.focus(); // expects only one such element on one page. If there are many, then the last one gets the focus.
-    el.setSelectionRange(el.value.length, el.value.length);
-  }
-}