polyfills: add Promise.withResolver

src/platform/polyfills.ts

@@ -15,3 +15,24 @@ if (typeof window.crypto.randomUUID === "undefined") {
       ) as `${string}-${string}-${string}-${string}-${string}`;
     };
 }
+
+
+if (typeof Promise.withResolvers === "undefined") {
+  // Chrome/Edge 119, Firefox 121, Safari/iOS 17.4
+
+  // Promise.withResolvers is generic and works with subclasses - the typescript built-in decl
+  // could not handle the subclassing case.
+  Promise.withResolvers = function <T>(this: AnyPromiseConstructor<T>) {
+    let resolve!: PromiseWithResolvers<T>["resolve"], reject!: PromiseWithResolvers<T>["reject"];
+    // These variables are expected to be set after `new this()`
+
+    const promise = new this((resolve0, reject0) => {
+      resolve = resolve0;
+      reject = reject0;
+    })
+
+    return {
+      promise, resolve, reject
+    }
+  }
+}