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.

vitest.config.ts

@@ -4,8 +4,8 @@ import {stringPlugin} from 'vite-string-plugin';
 
 export default defineConfig({
   test: {
-    include: ['web_src/**/*.test.js'],
-    setupFiles: ['web_src/js/vitest.setup.js'],
+    include: ['web_src/**/*.test.ts'],
+    setupFiles: ['web_src/js/vitest.setup.ts'],
     environment: 'happy-dom',
     testTimeout: 20000,
     open: false,