Change the bindings generator to emit enum definitions into the
Bindings/<Thing>.h file owned by the IDL that defines them, including
support IDLs that do not have a top-level interface or namespace.
Allow enum-only modules to generate bindings headers and make generated
bindings code include those owner headers when another IDL depends on
their enums.
This allows us to replace the hand made NavigationType.h header with
one generated by bindings.
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
