Move `Subjects` helper to `@casl/ability` from `@casl/prisma`

Been 2 years since I worked with CASL, but my new company is implementing authorization now, and I'd like to use CASL again. Except backend is in Python this time, so no Prisma for me =P

It seems to me that Subjects helper basically simplifies what's done in Infer subject types from interfaces docs. So it's applicable to main package as much as to Prisma.