Consider deprecating {} slots

[crcn]

In favor of HTML slots. Main reason for this is because of visual tooling.

Considerations:

• Conditional slots
• Conditional elements

For conditional elements, we can do this:

<div visible:variant>
  I'm only visible when variant is defined
</div>

Slot syntax:

<div component as="Test">
  <slot name="children">
    default children
  </slot> 
  <slot name="something-else">
  </slot>
</div>

<Test>
  default children

  <span visible:variant slot="something-else">
    child
  </span>
</Test>