Instead of checking if every single cell overrides the "must survive GC"
virtual, we can make this a HeapBlock level thing.
This avoids almost an entire GC heap traversal during the mark phase.
Previously, they would stay open for the entire WebContent lifetime,
or until the server closed the connection. This was particularly
noticeable on collaborative websites/games such as
https://jigsawpuzzles.io/, where the user using Ladybird would stick
around even after they had navigated away.
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
