First check if a string is a member of the enum before attempting
numeric conversion. This generates correct code for fields like:
AudioContextOptions {
latencyHint: AudioContextLatencyCategory | double;
}
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
