Create a Prisma file RFC

[sodic]

We are looking to move entities out of the Wasp file and into a dedicated Prisma file. Benefits of doing this are: tooling support, better Prisma feature support (enums, generators, datasources etc.), less maintenance in the Wasp compiler (maybe).

There are many open questions:

• how does the Wasp compiler get info about entities then?
• what is supported by Wasp in the Prisma file?
• where and how do we handle migrations?
• how do users run Prisma commands?

That's why we'll start first with an RFC and then from that we expect to create a detailed action plan on how to achieve this.

[sodic]

More motivation: https://discord.com/channels/686873244791210014/867713251479519232/1237475645290971136