CDATASection inherits from Text, and so it was incorrect for them to
claim not to be Text nodes.
This fixes at least two WPT subtests. :^)
It also exposed a bug in the DOM Parsing and Serialization spec,
where we're not told how to serialize CDATASection nodes.
Spec bug: https://github.com/w3c/DOM-Parsing/issues/38
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
The main motivation behind this is to remove JS specifics of the Realm
from the implementation of the Heap.
As a side effect of this change, this is a bit nicer to read than the
previous approach, and in my opinion, also makes it a little more clear
that this method is specific to a JavaScript Realm.
