We achieve this by keeping track of the number of HTMLSlotElements
inside each ShadowRoot (do via ad-hoc insertion and removal steps.)
This allows slottables assignment to skip over entire shadow roots when
we know they have no slots anyway.
Massive speedup on https://wpt.fyi/ which no longer takes minutes/hours
to load, but instead a "mere" 19 seconds. :^)
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
