This will allow us to use the GC to manage the lifetime of objects
that are not C++ objects, such as Swift objects. In the future we
could expand this cursed FFI to other languages as well.
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
Instead, smuggle it in as a `void*` private data and let Javascript
aware code cast out that pointer to a VM&.
In order to make this split, rename JS::Cell to JS::CellImpl. Once we
have a LibGC, this will become GC::Cell. CellImpl then has no specific
knowledge of the VM& and Realm&. That knowledge is instead put into
JS::Cell, which inherits from CellImpl. JS::Cell is responsible for
JavaScript's realm initialization, as well as converting of the void*
private data to what it knows should be the VM&.
GC-allocated objects should never have JS::SafeFunction/JS::Handle
fields.
For now the plugin only emits warnings here, as there are many cases
of this occurring in the codebase that aren't trivial to fix. It is also
behind a CMake flag since it is a _very_ loud warning.
Now that the lambda capture plugin isn't full of false-positives, we can
make the jump and start halting builds for these errors. It also allows
these plugins to be useful in CI.
