feat(RAC): expose --page-scroll-left from Modal

[lixiaoyan]

May supersede #9318

This allows us to properly place a dialog in a horizontally scrolled page (document.scrollingElement.scrollLeft !== 0).

<ModalOverlay className="absolute top-0 left-(--page-scroll-left) h-(--page-height) w-full">
  <Modal className="grid place-items-center sticky top-0 h-(--visual-viewport-height) w-full">
    <Dialog />
  </Modal>
</ModalOverlay>

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

🧢 Your Project:

[lixiaoyan]

Perhaps we should listen for window.onscroll while the Modal is open.

Tested, no need for that.