getWeekOfYear changes week number on Sunday - is this ISO?

**Describe the bug**
Calling `getWeekOfYear("2023-08-28")` returns 35, I would have expected 34.

It all comes down to whether we consider when the week starts, and I thought the ISO standard was to start on Monday.
Am I wrong?

I don't necessarily want things to change, but I wondered whether it was a bug, and will this behavior ever change, or can it be related to the device (browser locale, etc.)

Start with getWeekOfYear and the date 2023-08-28; verify the week-start and locale assumptions behind the returned value. Done means the expected behavior is decided—ISO Monday-based or current behavior—and that decision is reflected in the relevant API guidance.

Written by the indexing model from the issue text.