[@rancher/components] Card.vue: Toggle horizontal rule

[rak-phillip]

The default card styles contain a horizontal rule between the title and card body. This horizontal rule isn't always preferred in card usage so it would be nice if we could disable when the consumer desires.

It might make sense to shift the horizontal rule into the title slot and allow the consumer to override via slot usage. However, taking this approach might break existing cards that utilize the slot to generate a title so it might be better to introduce a prop that allows for the toggle.

🔳 Acceptance Criteria

• [ ] When the horizontal rule is disabled, there should be no visible line between the card title and body.
• [ ] When the horizontal rule is enabled, the card should have the default horizontal rule as defined in the current styles.
• [ ] The toggle for the horizontal rule should be easily accessible to the consumer.
• [ ] Enable the horizontal rule by default.
• [ ] The code changes should be backwards compatible and not break any existing functionality.
• [ ] Update storybook to include information about the new toggle feature and how to use it.
• [ ] Add unit tests to ensure the toggle works as expected.
• [ ] The toggle should not affect any other styles or functionality of the card component.

[gaktive]

Depends on what's decided on rancher/dashboard#8859 for the overall approach to cards. That's lined up for Q3.