text: Title: Add support for including count of relational fields
  Body: ## Problem
  
  Prisma supports including the count of a relational field, so we should too.
  
  ```js
  const users = await prisma.user.findMany({
    include: {
      _count: {
        select: { posts: true },
      },
    },
  })
  ```
  which returns an object like this
  ```js
  {
    id: 1,
    email: 'alice@prisma.io',
    name: 'Alice',
    _count: { posts: 2 }
  }
  ```
  
  Prisma also have support for filtering by the count of the relation, see https://www.prisma.io/docs/concepts/components/prisma-client/aggregation-grouping-summarizing#filter-the-relation-count