Updates Wasp AI to work with the Prisma file

[infomiho]

Update the Wasp AI features to generate models in a separate Prisma file. Continuation of #2035

• I've renamed mentions of entity to model where applicable so the GPT would understand better that we are talking about Prisma models.
• I've used the existing Prisma format to fix the Prisma file if needed after it's generated.
• Fixed some minor stuff along the way.