supported_property_names()
Instead, take advantage of `ElementByIdMap` and `m_potentially_named_elements` to gather named elements more efficiently.
This makes some common types like JS::Object smaller (by 8 bytes) and yields a minor speed improvement on many benchmarks.
With this change we maintain a data structure that maps ids to corresponding elements. This allows us to avoid tree traversal in getElementById() in all cases except ones when lookup happens for unconnected elements.
