issue-855: AlertDialog add support for allowsKeyboardConfirmation prop

[lishichengyan]

Closes https://github.com/adobe/react-spectrum/issues/855

✅ Pull Request Checklist:

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

📝 Test Instructions:

Open an AlertDialog, verify if pressing the "Enter" key closes it and triggers onPrimaryAction.

🧢 Your Project: