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/fetch.test.ts

@@ -0,0 +1,10 @@
+import {GET, POST, PATCH, PUT, DELETE} from './fetch.ts';
+
+// tests here are only to satisfy the linter for unused functions
+test('exports', () => {
+  expect(GET).toBeTruthy();
+  expect(POST).toBeTruthy();
+  expect(PATCH).toBeTruthy();
+  expect(PUT).toBeTruthy();
+  expect(DELETE).toBeTruthy();
+});