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/HTML/DOMStringMap.cpp

@@ -185,7 +185,7 @@ bool DOMStringMap::delete_existing_named_property(DeprecatedString const& name)
 
 JS::Value DOMStringMap::named_item_value(FlyString const& name) const
 {
-    return js_string(vm(), determine_value_of_named_property(name));
+    return JS::PrimitiveString::create(vm(), determine_value_of_named_property(name));
 }
 
 }