Add async setImmediate util

Adds an async/promise-based version of `setImmediate`. Note that, despite being
poorly adopted, `setImmediate` is polyfilled, and should be more performant
than `sleep(0)`.

Signed-off-by: Clark Fischer <clark.fischer@gmail.com>

spec/unit/utils.spec.ts

@@ -1,3 +1,19 @@
+/*
+Copyright 2023 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
 import * as utils from "../../src/utils";
 import {
     alphabetPad,
@@ -587,4 +603,22 @@ describe("utils", function () {
             expect(utils.isSupportedReceiptType("this is a receipt type")).toBeFalsy();
         });
     });
+
+    describe("sleep", () => {
+        it("resolves", async () => {
+            await utils.sleep(0);
+        });
+
+        it("resolves with the provided value", async () => {
+            const expected = Symbol("hi");
+            const result = await utils.sleep(0, expected);
+            expect(result).toBe(expected);
+        });
+    });
+
+    describe("immediate", () => {
+        it("resolves", async () => {
+            await utils.immediate();
+        });
+    });
 });