Corresponds to https://github.com/whatwg/html/pull/10683
As part of this, I noticed we incorrectly were setting the "is popup"
flag on the Navigable instead of the BrowsingContext. I've fixed that
and removed the erroneous flag from Navigable.
To check whether a NavigationParams is null, we have to check whether
it's `Empty` or `NullWithError`. Instead, we can merge both of these
possible variants into an optional error. If `NullOrError` has no
value it's null, otherwise it contains an error message.
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
