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/dom.test.ts

@@ -0,0 +1,16 @@
+import {createElementFromAttrs, createElementFromHTML} from './dom.ts';
+
+test('createElementFromHTML', () => {
+  expect(createElementFromHTML('<a>foo<span>bar</span></a>').outerHTML).toEqual('<a>foo<span>bar</span></a>');
+});
+
+test('createElementFromAttrs', () => {
+  const el = createElementFromAttrs('button', {
+    id: 'the-id',
+    class: 'cls-1 cls-2',
+    'data-foo': 'the-data',
+    disabled: true,
+    required: null,
+  });
+  expect(el.outerHTML).toEqual('<button id="the-id" class="cls-1 cls-2" data-foo="the-data" disabled=""></button>');
+});