Add a sleep utility function.

source/utilities/exports.ts

@@ -7,6 +7,7 @@ export * from "./http.js";
 export * from "./logging.js";
 export * from "./query-selectors.js";
 export * from "./report-a-bug.js";
+export * from "./sleep.js";
 export * from "./text.js";
 export * from "./user.js";
 export * from "./validators.js";

source/utilities/sleep.ts

@@ -0,0 +1,9 @@
+/**
+ * Promisified {@linkcode window.setTimeout}.
+ * @param timeout The amount of time in milliseconds to sleep for.
+ */
+export async function sleep(timeout: number): Promise<void> {
+  return new Promise((resolve) => {
+    window.setTimeout(resolve, timeout);
+  });
+}