There was a bug in the HTML proposal where a synthetic realm settings
object's principal realm was a shadow realm if there were nested shadow
realms, which this assertion catches more directly (rather than later
down the track, where it is used).
We were meant to also assert for this case, but we were previously
returning early.
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
