LibJS: Use IntrusiveList for Allocator's block lists

This way we don't need to deal with shifting vector storage, and most
operations are upgraded from O(n) to O(1) :^)

Libraries/LibJS/Heap/HeapBlock.h

@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include <AK/IntrusiveList.h>
 #include <AK/Types.h>
 #include <LibJS/Forward.h>
 #include <LibJS/Runtime/Cell.h>
@@ -79,6 +80,8 @@ public:
         return cell(cell_index);
     }
 
+    IntrusiveListNode m_list_node;
+
 private:
     HeapBlock(Heap&, size_t cell_size);