LibWeb: Introduce the slottable concept for DOM elements and text nodes

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.

Userland/Libraries/LibWeb/DOM/Text.cpp

@@ -32,6 +32,8 @@ void Text::initialize(JS::Realm& realm)
 void Text::visit_edges(Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);
+    SlottableMixin::visit_edges(visitor);
+
     visitor.visit(dynamic_cast<JS::Cell*>(m_owner.ptr()));
 }