Add base and theme aware tokens for splitter bar

[jattasNI]

Pull Request

🤨 Rationale

We are working on making a resizable chat pane. Brandon provided guidance about how to style the draggable splitter bar. Some of the colors are not yet present in the design system.

👩‍💻 Implementation

1. Add a new base token for Black77_100
2. Add a new theme-aware token for splitter background color.
3. Update description of divider background color to distinguish it from splitter

🧪 Testing

Storybook inspection

✅ Checklist

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