LibJS: Expose minimum possible cell size of JS::Heap

Use this to avoid creating a 16 byte cell allocator on x86_64, where the
size of FreelistEntry is 24 bytes. Every JS::Cell must be at least the
size of the FreelistEntry or things start crashing, so the 16 byte
allocator was wasted on that platform.

Userland/Libraries/LibJS/Heap/HeapBlock.h

@@ -101,6 +101,9 @@ private:
     size_t m_next_lazy_freelist_index { 0 };
     FreelistEntry* m_freelist { nullptr };
     alignas(Cell) u8 m_storage[];
+
+public:
+    static constexpr size_t min_possible_cell_size = sizeof(FreelistEntry);
 };
 
 }