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/LibWeb/DOM/DOMTokenList.cpp

@@ -259,7 +259,7 @@ JS::Value DOMTokenList::item_value(size_t index) const
     auto const& string = item(index);
     if (string.is_null())
         return JS::js_undefined();
-    return JS::js_string(vm(), string);
+    return JS::PrimitiveString::create(vm(), string);
 }
 
 }