feat(RAC): expose --page-width and --visual-viewport-width from Modal

[lixiaoyan]

Closes

This allows us to properly place a dialog with horizontal scroll document.

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

[snowystinger]

Seems reasonable

[lixiaoyan]

@devongovett Hi, any chance to get this merged before next release? This is necessary for our project.