LibJS: Add a basic mark&sweep garbage collector :^)

Objects can now be allocated via the interpreter's heap. Objects that
are allocated in this way will need to be provably reachable from at
least one of the known object graph roots.

The roots are currently determined by Heap::collect_roots().

Anything that wants be collectable garbage should inherit from Cell,
the fundamental atom of the GC heap.

This is pretty neat! :^)

Libraries/LibJS/Interpreter.h

@@ -28,6 +28,7 @@
 
 #include <AK/Vector.h>
 #include <LibJS/Forward.h>
+#include <LibJS/Heap.h>
 
 namespace JS {
 
@@ -45,12 +46,16 @@ public:
     Object& global_object() { return *m_global_object; }
     const Object& global_object() const { return *m_global_object; }
 
+    Heap& heap() { return m_heap; }
+
     void do_return();
 
 private:
     void enter_scope(const ScopeNode&);
     void exit_scope(const ScopeNode&);
 
+    Heap m_heap;
+
     Vector<ScopeFrame> m_scope_stack;
 
     Object* m_global_object { nullptr };