LibJS: Replace standalone js_string() with PrimitiveString::create()

Note that js_rope_string() has been folded into this, the old name was
misleading - it would not always create a rope string, only if both
sides are not empty strings. Use a three-argument create() overload
instead.

Userland/Libraries/LibJS/Runtime/Object.cpp

@@ -961,7 +961,7 @@ ThrowCompletionOr<MarkedVector<Value>> Object::internal_own_property_keys() cons
     // 2. For each own property key P of O such that P is an array index, in ascending numeric index order, do
     for (auto& entry : m_indexed_properties) {
         // a. Add P as the last element of keys.
-        keys.append(js_string(vm, DeprecatedString::number(entry.index())));
+        keys.append(PrimitiveString::create(vm, DeprecatedString::number(entry.index())));
     }
 
     // 3. For each own property key P of O such that Type(P) is String and P is not an array index, in ascending chronological order of property creation, do