refactor: flush microtask queue implementation

Author: mdjastrzebski

src/flushMicroTasks.ts

@@ -1,13 +1,5 @@
 import { setImmediate } from './helpers/timers';
 
-type Thenable<T> = { then: (callback: () => T) => unknown };
-
-export function flushMicroTasks(): Thenable<void> {
-  return {
-    // using "thenable" instead of a Promise, because otherwise it breaks when
-    // using "modern" fake timers
-    then(resolve) {
-      setImmediate(resolve);
-    },
-  };
+export function flushMicroTasks() {
+  return new Promise((resolve) => setImmediate(resolve));
 }