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/webcomponents/absolute-date.test.ts

@@ -0,0 +1,15 @@
+import {toAbsoluteLocaleDate} from './absolute-date.ts';
+
+test('toAbsoluteLocaleDate', () => {
+  expect(toAbsoluteLocaleDate('2024-03-15', 'en-US', {
+    year: 'numeric',
+    month: 'long',
+    day: 'numeric',
+  })).toEqual('March 15, 2024');
+
+  expect(toAbsoluteLocaleDate('2024-03-15', 'de-DE', {
+    year: 'numeric',
+    month: 'long',
+    day: 'numeric',
+  })).toEqual('15. März 2024');
+});