Create a submodule enabling user registration resources

We're needing to create a Register and a "Me" endpoint in JSON:API. This was discussed previously in #2877549: How to register a new user using JSON API module → . It was marked as "something JSON:API doesn't do."

But, we now have JSON:API Resources! Which means we can do this. I was considering a jsonapi_resources_user or jsonapi_user module. This would allow us to have /jsonapi/user/register and /jsonapi/me. The latter returning information about the current user. The registration resource would be a port UserRegistrationResource to a JSON:API resource.

I'm torn if this should be its own project (providing two resources) or a submodule. But, it does make sense as a submodule since it is very specific. And this way the JSON:API Resources module can provide a collection of very common and specific features as resources.