LibJS: Resolve a circular include problem between HeapBlock and Cell

Cell::heap() and Cell::vm() needed to access member functions from
HeapBlock, and wanted to be inline, so they were moved to VM.h.
That approach will no longer work with VM.h not being included in every
file (starting from the next commit), so this commit fixes that circular
import issue by introducing secondary base classes to host the
references to Heap and VM, respectively.

Userland/Libraries/LibJS/Heap/Heap.h

@@ -19,12 +19,13 @@
 #include <LibJS/Heap/Cell.h>
 #include <LibJS/Heap/CellAllocator.h>
 #include <LibJS/Heap/Handle.h>
+#include <LibJS/Heap/Internals.h>
 #include <LibJS/Heap/MarkedVector.h>
 #include <LibJS/Runtime/WeakContainer.h>
 
 namespace JS {
 
-class Heap {
+class Heap : public HeapBase {
     AK_MAKE_NONCOPYABLE(Heap);
     AK_MAKE_NONMOVABLE(Heap);
 
@@ -57,8 +58,6 @@ public:
 
     void collect_garbage(CollectionType = CollectionType::CollectGarbage, bool print_report = false);
 
-    VM& vm() { return m_vm; }
-
     bool should_collect_on_every_allocation() const { return m_should_collect_on_every_allocation; }
     void set_should_collect_on_every_allocation(bool b) { m_should_collect_on_every_allocation = b; }
 
@@ -106,8 +105,6 @@ private:
 
     bool m_should_collect_on_every_allocation { false };
 
-    VM& m_vm;
-
     Vector<NonnullOwnPtr<CellAllocator>> m_allocators;
 
     HandleImpl::List m_handles;