It's possible to parse an `@import` rule that isn't attached to a
document. We only actually need it to have one when fetching the linked
style sheet, and that should only happen when the CSSImportRule is
attached to a document. So, we can just accept a null pointer when
constructing it.
We relied on that Document to get the Realm, so pass that in as a
separate parameter.
To prepare for introducing a CSS::URL type, we need to qualify any use
of LibURL as `::URL::foo` instead of `URL::foo` so the compiler doesn't
get confused.
Many of these uses will be replaced, but I don't want to mix this in
with what will likely already be a large change.
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
