Demix Slottable

[Elchi3]

Slottable is a small mixin:

interface mixin Slottable {
  readonly attribute HTMLSlotElement? assignedSlot;
};
Element includes Slottable;
Text includes Slottable;

I think the assignSlot property can be added to Element and Text directly.
Here again we notice how inconsistent things are right now: api/Text.json already has assignedSlot and even MDN docs exist at that place: https://developer.mozilla.org/en-US/docs/Web/API/Text/assignedSlot. So, half of the work is done.

This PR now removes the Slottable.json mixin file and moves the assignedSlot data to Element.json.

[ddbeck]

This is interesting, as I guess it's the first instance we have of the small mixin option in the new guideline.

Feels tidier already. Thank you! 🎉