Add a title slot to the Card component

[kjohn1922]

Pull Request

Add a title slot for the nimble-card component.

See #296 and the nimble-card spec for more information.

🤨 Rationale

The title slot in the nimble-card component will make it easy for clients to add a title with the correct styling and to enforce consistency across usages.

The style itself is still a placeholder. We'll apply styles from a visual design in a future PR, before releasing the Routines UI with the card component.

👩‍💻 Implementation

• Add a slot with name="title" to the nimble-card template
• Add a style to apply a title font to the title slot (placeholder, may require additional styling from visual design)
• For accessibility, put a section at the root of the component template
  • Use display: contents to allow the host display settings to apply directly to the slotted content.
• Add a title to the matrix stories
• Add a configurable title to the storybook page.
  • Modeled after the nimble-banner with similar description text for the title.

🧪 Testing

Updated storybook and matrix tests:

✅ Checklist

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