feat(card): add primary action component

[tychenjiajun]

#287

[matsp]

@tychenjiajun In the case of switching from normal card to primary action card at runtime, I would have to duplicate the component and render conditionally by a prop?

[tychenjiajun]

@matsp No, you just need to duplicate the contents in default slot.

[matsp]

@tychenjiajun Didn't see the benefit yet. The user will experience more markup. To have it all in one component is more complex for us but the user will benefit from it by writing less. Maybe I am missing something. Can you describe your idea to split it? I just want to be sure to find the right balance between development and end user experience 😅

[matsp]

After reading the issue this PR make sense!

[tychenjiajun]

@matsp And considering accessibility, it's hard to bind a keydown event to the actionable content.
https://codepen.io/chankcccc/pen/qzNKyb
https://codepen.io/chankcccc/pen/MMeXeq

[tychenjiajun]

@matsp We can also keep an actionable prop and use conditional rendering to provide a better user experience. When actionable is true, render a built-in mdc-card__primary-action and bind all the event listeners to it. In this way, users can choose either actionable prop or an independent and highly customizable <m-card-primary-action> component.