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/utils/time.test.ts

@@ -0,0 +1,15 @@
+import {startDaysBetween} from './time.ts';
+
+test('startDaysBetween', () => {
+  expect(startDaysBetween(new Date('2024-02-15'), new Date('2024-04-18'))).toEqual([
+    1708214400000,
+    1708819200000,
+    1709424000000,
+    1710028800000,
+    1710633600000,
+    1711238400000,
+    1711843200000,
+    1712448000000,
+    1713052800000,
+  ]);
+});