Using the new hooks in the XML Parser's listener interface, we now
append DOM nodes for CDATASections and ProcessingInstructions
to the document as they are encountered. This commit also fixes where
comment nodes are appended, ensuring they are added to the current node
instead of the document root.
When the XML parser appends child nodes to a template element, it must
actually append the template element's contents. This special behavior
caused us to return to the wrong parent element after adding child
nodes to a template element, leading to a crash.
Names like ":hi", "wow:", or "a🅱️c" are invalid, so early-out instead
of searching our namespace stack for matching prefixes.
And also rename the function because it's relevant to attributes too.
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
