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/modules/toast.test.ts

@@ -0,0 +1,16 @@
+import {showInfoToast, showErrorToast, showWarningToast} from './toast.ts';
+
+test('showInfoToast', async () => {
+  showInfoToast('success 😀', {duration: -1});
+  expect(document.querySelector('.toastify')).toBeTruthy();
+});
+
+test('showWarningToast', async () => {
+  showWarningToast('warning 😐', {duration: -1});
+  expect(document.querySelector('.toastify')).toBeTruthy();
+});
+
+test('showErrorToast', async () => {
+  showErrorToast('error 🙁', {duration: -1});
+  expect(document.querySelector('.toastify')).toBeTruthy();
+});