101Implement a Promise library with .then(), .catch(), chaining, and asynchronous handler scheduling

Implement class MyPromise that follows the Promises/A+ rules, without using the built-in Promise anywhere in your implementation:

• new MyPromise((resolve, reject) => …): the executor runs synchronously; if it throws, the promise rejects. A promise settles once; later calls are ignored.
• then(onFulfilled, onRejected) returns a new promise. Handlers run asynchronously (in a microtask, use queueMicrotask), in the order they were registered, even if the promise is already settled.
• A handler's return value resolves the next promise; a thrown error rejects it. Missing handlers pass values and errors through.
• Resolution procedure: resolving with a thenable (any object or function with a then method) adopts its state; resolving a promise with itself rejects with a TypeError; a thenable's callbacks are honoured only once.
• catch(onRejected), finally(onFinally) (runs on both outcomes, passes the original value or error through unless it throws), and static MyPromise.resolve and MyPromise.reject.

new MyPromise((resolve) => setTimeout(() => resolve(2), 10))
  .then((n) => n * 10)
  .then((n) => { throw new Error(`too big: ${n}`); })
  .catch((e) => e.message)
  .finally(() => console.log("done"))
  .then(console.log); // "too big: 20"