Create enum for z-index values

[m-akinc]

Pull Request

🤨 Rationale

Fixes #539

👩‍💻 Implementation

• Created a new enum in utilities with a few preset z-index values.
• Updated stylesheets for anchored-region, dropdown, and table to use enum values rather than hardcoded values.

🧪 Testing

None

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.

[m-akinc]

@atmgrifter00 could you please buddy this?