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
