Make a concept that indicates when reflect<> can be called on a type

[joshuamaiche]

Currently, reflectable<T> exists. However, its name is somewhat misleading. If a class has a meta<T> specialization, for instance, reflect<T> is safe to be called on the type. However, reflectable<T> specifically excludes glaze_object_t<T>, which will be true if a class has a specialized meta<T>.

If a developer knows about this, it's an inconvenience. Cases like to<JSON, T> have to check for both reflectable<T> and glaze_object_t<T>. If a developer doesn't know about this, it's way worse, as they will think they're catching all the relevant types with requires reflectable<T>, but are actually missing all the types that have a meta<T> specialization.

Assuming reflectable<T> can't be renamed since it might break others' code, I propose adding a has_reflect<T> concept and using that internally, as an example other devs can follow.