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.ts

@@ -0,0 +1,41 @@
+import {isObject} from '../utils.ts';
+
+const {csrfToken} = window.config;
+
+// safe HTTP methods that don't need a csrf token
+const safeMethods = new Set(['GET', 'HEAD', 'OPTIONS', 'TRACE']);
+
+// fetch wrapper, use below method name functions and the `data` option to pass in data
+// which will automatically set an appropriate headers. For json content, only object
+// and array types are currently supported.
+export function request(url, {method = 'GET', data, headers = {}, ...other} = {}) {
+  let body, contentType;
+  if (data instanceof FormData || data instanceof URLSearchParams) {
+    body = data;
+  } else if (isObject(data) || Array.isArray(data)) {
+    contentType = 'application/json';
+    body = JSON.stringify(data);
+  }
+
+  const headersMerged = new Headers({
+    ...(!safeMethods.has(method) && {'x-csrf-token': csrfToken}),
+    ...(contentType && {'content-type': contentType}),
+  });
+
+  for (const [name, value] of Object.entries(headers)) {
+    headersMerged.set(name, value);
+  }
+
+  return fetch(url, {
+    method,
+    headers: headersMerged,
+    ...other,
+    ...(body && {body}),
+  });
+}
+
+export const GET = (url, opts) => request(url, {method: 'GET', ...opts});
+export const POST = (url, opts) => request(url, {method: 'POST', ...opts});
+export const PATCH = (url, opts) => request(url, {method: 'PATCH', ...opts});
+export const PUT = (url, opts) => request(url, {method: 'PUT', ...opts});
+export const DELETE = (url, opts) => request(url, {method: 'DELETE', ...opts});