non Portaled confirm() dialogs are unstyled.

### Description

In https://github.com/primer/react/pull/3840 we're no longer using `<Portal>` to place the dialog, but doing so we realised that the `<BaseStyles>` provider is missing from the `confirm()` call.

### Steps to reproduce

1. Remove `<Portal>` from `<Dialog>`.
2. Call `confirm()`
3. Observe unstyled `<Dialog>`

### Version

v36.4.0

### Browser

_No response_