introduce promiseWithResolvers helper (graphql#3902)

Promise.withResolvers is currently a [stage 2 proposal for
JS](https://github.com/tc39/proposal-promise-with-resolvers). This adds
an equivalent jsutil to standardize usage.

Author: robrichard

src/jsutils/__tests__/promiseWithResolvers-test.ts

@@ -0,0 +1,21 @@
+import { expect } from 'chai';
+import { describe, it } from 'mocha';
+
+import { expectPromise } from '../../__testUtils__/expectPromise.js';
+
+import { promiseWithResolvers } from '../promiseWithResolvers.js';
+
+describe('promiseWithResolvers', () => {
+  it('resolves values', async () => {
+    const { promise, resolve } = promiseWithResolvers();
+    resolve('foo');
+    expect(await expectPromise(promise).toResolve()).to.equal('foo');
+  });
+
+  it('rejects values', async () => {
+    const { promise, reject } = promiseWithResolvers();
+    const error = new Error('rejected');
+    reject(error);
+    await expectPromise(promise).toRejectWith('rejected');
+  });
+});

src/jsutils/promiseWithResolvers.ts

@@ -0,0 +1,20 @@
+import type { PromiseOrValue } from './PromiseOrValue.js';
+
+/**
+ * Based on Promise.withResolvers proposal
+ * https://github.com/tc39/proposal-promise-with-resolvers
+ */
+export function promiseWithResolvers<T>(): {
+  promise: Promise<T>;
+  resolve: (value: T | PromiseOrValue<T>) => void;
+  reject: (reason?: any) => void;
+} {
+  // these are assigned synchronously within the Promise constructor
+  let resolve!: (value: T | PromiseOrValue<T>) => void;
+  let reject!: (reason?: any) => void;
+  const promise = new Promise<T>((res, rej) => {
+    resolve = res;
+    reject = rej;
+  });
+  return { promise, resolve, reject };
+}