152. Promise.finally Polyfill

Implement the behavior of Promise.prototype.finally as a standalone helper.

Implement:

function promiseFinally(promise, onFinally) {}

Behavior should match native finally semantics:

• Run onFinally after promise settles (both resolve and reject)
• If original promise resolves, pass through original value
• If original promise rejects, pass through original reason
• If onFinally throws or returns a rejected promise, reject with that new error
• If onFinally is not a function, behave like pass-through

Do not use Promise.prototype.finally in your implementation.