add: readonly attributes for CalendarCell

[Persists]

Found During Testing

The CalendarCell component was missing the readonly attributes; this PR adds those.

Changes:

• Adjusted the useCalendarCell hook.
• Updated the RAC component to apply:
  • data-readonly on the button within the grid cell.
  • aria-readonly on the grid cell element.
• Added a test

Let me know if there are any further adjustments needed 😃

✅ 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:

[Persists]

The data-readonly attribute is currently also missing on the Calendar and RangeCalendar components. I can include it in this PR as well.

[yihuiliao]

Did you have a specific use case where this came up? I think it might be weird to be able to make only some cells isReadOnly.

[yihuiliao]

Closing due to inactivity. Happy to revisit this if we can get additional info on the use case!