LibJS: Fast-path own-property enumeration and reduce descriptor lookups

Before this change, PropertyNameIterator (used by for..in) and
`Object::enumerable_own_property_names()` (used by `Object.keys()`,
`Object.values()`, and `Object.entries()`) enumerated an object's own
enumerable properties exactly as the spec prescribes:
- Call `internal_own_property_keys()`, allocating a list of JS::Value
  keys.
- For each key, call internal_get_own_property() to obtain a
  descriptor and check `[[Enumerable]]`.

While that is required in the general case (e.g. for Proxy objects or
platform/exotic objects that override `[[OwnPropertyKeys]]`), it's
overkill for ordinary JS objects that store their own properties in the
shape table and indexed-properties storage.

This change introduces `for_each_own_property_with_enumerability()`,
which, for objects where
`eligible_for_own_property_enumeration_fast_path()` is `true`, lets us
read the enumerability directly from shape metadata (and from
indexed-properties storage) without a per-property descriptor lookup.
When we cannot avoid `internal_get_own_property()`, we still
benefit by skipping the temporary `Vector<Value>` of keys and avoiding
the unnecessary round-trip between PropertyKey and Value.

Libraries/LibWeb/HTML/WindowProxy.h

@@ -33,6 +33,8 @@ public:
     virtual JS::ThrowCompletionOr<bool> internal_delete(JS::PropertyKey const&) override;
     virtual JS::ThrowCompletionOr<GC::RootVector<JS::Value>> internal_own_property_keys() const override;
 
+    virtual bool eligible_for_own_property_enumeration_fast_path() const override final { return false; }
+
     GC::Ptr<Window> window() const { return m_window; }
     void set_window(GC::Ref<Window>);