This change makes Ladybird conform to the current requirements at
https://w3c.github.io/core-aam/#roleMappingComputedRole in the “Core
Accessibility API Mappings” spec for the case of “orphaned” li elements;
that is, any li element which doesn’t have a role=list ancestor.
The core-aam spec requires that in such cases, the li element must not
be assigned the “listitem” role but instead must be treated as if it had
no role at all.
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
Now that we have RTTI in userspace, we can do away with all this manual
hackery and use dynamic_cast.
We keep the is<T> and downcast<T> helpers since they still provide good
readability improvements. Note that unlike dynamic_cast<T>, downcast<T>
does not fail in a recoverable way, but will assert if the object being
casted is not a T.
