Add support for fixed week display in Calendar

[ryo-manba]

Closes https://github.com/adobe/react-spectrum/issues/4789, https://github.com/adobe/react-spectrum/issues/5840

I added props to the Calendar to enable the selection of a fixed 6-week view.

✅ 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.
• [ ] Updated documentation (if it already exists for this component).
• [ ] Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

1. Visit the Date Picker example in your local Storybook.
2. Go to the Controls tab and set isFixedWeeks to true.
3. Observe that the calendar now consistently displays 6 weeks.
4. Confirm the 6-week display at the DateRangePicker example as well.

🧢 Your Project: