Shared workers are essentially just workers that may be accessed from
scripts within the same origin. There are plenty of FIXMEs here (mostly
building on existing worker FIXMEs that are already in place), but this
lets us run the shared worker variants of WPT tests.
This is to differentiate the agent representation for the parent
process for the WorkerAgent in the child process which is actually
hooked up to the javascript VM.
I am not sure if this is a good name, but I can't really think of
anything better which is consistent with the names used by the rest
of the codebase.
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
