This is a GC-aware wrapper around AK::HashMap. Entry values are treated
as GC roots, much like the GC::RootVector we already had.
We also provide GC::OrderedRootHashMap as a convenience.
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
