TimerSerenity was the only implementation of the abstract Timer base
class. It simply wrapped a Core::Timer and delegated every method.
Remove the virtual dispatch by making Timer concrete and absorbing
the TimerSerenity implementation directly. Timer::create() now
allocates on the GC heap directly instead of going through
EventLoopPlugin::create_timer().
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
