For example, on https://html.spec.whatwg.org, there are hundreds of
thousands of nodes. This method is invoked as each node is inserted.
Traversing the entire tree each time takes a prohibitively long time,
so let's bail early when we know the operation is a no-op.
This implements looking up a slottable's assigned slot, and a slot's
list of assigned slottables. For the latter, only unflattened lookups
are implemented so far.
This implements automatic slottable assignment by way of hooking into
the element attribute change steps. When the `name` attribute of a slot
or the `slot` attribute of a slottable changes, assignment is performed.
This implements manual slottable assignment by way of HTMLSlotElement's
`assign` API. This includes all of the slottable-related AOs needed to
perform the assignment.
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.
