Polyfill Promise.withResolvers

Fixes: https://gitlab.com/soapbox-pub/soapbox/-/issues/1802

src/main.tsx

@@ -1,3 +1,5 @@
+import 'soapbox/polyfill/Promise.withResolvers.ts';
+
 import { enableMapSet } from 'immer';
 import { createRoot } from 'react-dom/client';
 

src/polyfill/Promise.withResolvers.ts

@@ -0,0 +1,14 @@
+if (!Promise.withResolvers) {
+  Promise.withResolvers = function withResolvers<T>(): PromiseWithResolvers<T> {
+
+    let resolve: (value: T | PromiseLike<T>) => void;
+    let reject: (reason?: any) => void;
+
+    const promise = new this<T>((_resolve, _reject) => {
+      resolve = _resolve;
+      reject = _reject;
+    });
+
+    return { resolve: resolve!, reject: reject!, promise };
+  };
+}