Data type JSON

When converting the following datamodel:
```
type A {
  id: ID! @unique

  text: String!

  meta: JSON!
}
```

Using the following method:
```
cat datamodel.graphql | prisma2 convert > schema.prisma
```

I get the following schema:
```
model A {
  id   String @default(cuid()) @id @unique
  text String
}
```

As we can see, the `meta: JSON` attribute is missing