Revert "LibJS: Remove "uprooting" mechanism from garbage collector"

This reverts commit 6232ad3a0d.

Unfortunately this introduced some flakiness on CI, so it wasn't
quite this simple.

Userland/Libraries/LibJS/Heap/Heap.h

@@ -76,6 +76,8 @@ public:
 
     BlockAllocator& block_allocator() { return m_block_allocator; }
 
+    void uproot_cell(Cell* cell);
+
 private:
     static bool cell_must_survive_garbage_collection(Cell const&);
 
@@ -110,6 +112,8 @@ private:
     MarkedVectorBase::List m_marked_vectors;
     WeakContainer::List m_weak_containers;
 
+    Vector<GCPtr<Cell>> m_uprooted_cells;
+
     BlockAllocator m_block_allocator;
 
     size_t m_gc_deferrals { 0 };