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
...{All} to ParentNode. Exposes createDocumentFragment and
createComment on Document. Stubs out the document.body setter.
Also adds ParentNode back :^).
This requires moving remove_all_children() from ParentNode to
Node, which makes ParentNode.cpp empty, so remove it.
It also co-opts the existing Node::text_content() method and
tweaks it slightly to fit the semantics of Node.textContent.
LibWeb keeps growing and the Web namespace is filling up fast.
Let's put DOM stuff into Web::DOM, just like we already started doing
with SVG stuff in Web::SVG.
