LibJS: Cache valid indices in IndexedPropertyIterator

Refetching the list of indices every time we increment the iterator was
showing up hot & heavy in a profile of Discord.

Userland/Libraries/LibJS/Runtime/IndexedProperties.h

@@ -106,6 +106,7 @@ private:
     void skip_empty_indices();
 
     const IndexedProperties& m_indexed_properties;
+    Vector<u32> m_cached_indices;
     u32 m_index;
     bool m_skip_empty;
 };