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/standalone/devtest.ts

@@ -1,4 +1,4 @@
-import {showInfoToast, showWarningToast, showErrorToast} from '../modules/toast.js';
+import {showInfoToast, showWarningToast, showErrorToast} from '../modules/toast.ts';
 
 function initDevtestToast() {
   const levelMap = {info: showInfoToast, warning: showWarningToast, error: showErrorToast};