feat(config): add `env` helper
Add a helper that safely checks if an environment variable is defined
and is not empty, and returns its value if so, otherwise throws an
error.
This allows users to avoid having boilerplate code related to
environment variables in their config files or using unsafe assertions
like `process.env.DATABASE_URL!`.
Usage:
```ts
import { defineConfig, env } from '@prisma/config'
export default defineConfig({
engine: 'classic',
datasource: {
url: env('DATABASE_URL'),
},
})
```
Closes: https://linear.app/prisma-company/issue/TML-1508/env-helper-in-prismaconfig