Allow using env vars for all generator options

[steebchen]

Defining env vars in the schema is only possible for fields which are known to the Prisma CLI, which means Go-specific options like package can only be hardcoded; using an env var does not work at this time.

generator client {
  provider = env("PRISMA_PROVIDER")
  output   = env("PRISMA_OUTPUT")
  package  = env("PRISMA_PACKAGE") // <- does not work at this time
}

related: #166

[elmarsan]

I'll work on it

[steebchen]

Env support would happen in the Prisma CLI I believe, so I don't think changing anything in the Go client can solve this.