A slottable is either a DOM element or a DOM text node. They may be
assigned to slots (HTMLSlotElement) either automatically or manually.
Automatic assignment occurs by matching a slot's `name` attribute to
a slottable's `slot` attribute. Manual assignment occurs by using the
slot's (not yet implemented) `assign` API.
This commit does not perform the above assignments. It just sets up the
slottable concept via IDL and hooks the slottable mixin into the element
and text nodes.
NewAKString is effectively the default for any new IDL interface, so
let's mark this as the default behavior. It also makes it much easier to
figure out whatever interfaces are still left to port over to new AK
String.
The intent is to use these to autogenerate prototype declarations for
Window and WorkerGlobalScope classes.
And the spec links are just nice to have :^)
