It was annoying to maintain two separate but almost identical functions
that gradually accumulated small differences over time. This change
replaces them with a single function that resolves either width or
height, depending on the specified dimension.
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
