Global Register types conflicting with each other

[Kareem-Medhat]

When creating two Lucia instances, each with different session and user tables, you face the issue of the two Register types conflicting with each other. I don't see the reason for using these global types instead of type generics. Let me know if I'm missing something and if not then I'm willing to create a PR with the required changes.

[pilcrowonpaper]

Regardless of the feasibility, it would likely be a breaking change. This issue will be fixed if we go with #1639 anyway