chore: export SelectBoxGroupContext

[yihuiliao]

just something i remembered when i was looking at SelectBox

also personally felt like SelectBoxContext was more of an internal context thing so i removed it from being exported

✅ 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:

[rspbot]

Build successful! 🎉

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## API Changes

@react-spectrum/s2

/@react-spectrum/s2:SelectBoxContext

-SelectBoxContext {
-  UNTYPED
-}

/@react-spectrum/s2:SelectBoxGroupContext

+SelectBoxGroupContext {
+  UNTYPED
+}

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