Support for EXISTS as a select expression

[joshkel]

Environment

Knex version: 2.4.2
Database + version: Postgres 14.7
OS: macOS

Feature discussion / request

I see that Knex supports whereExists and whereNotExists, but I don't see how to use EXISTS as a select column expression. For example, something like the following:

select *,
   exists (select * from order where order.person_id = people.id) as has_orders
   from people;

Perhaps the usage would look something like this (modeled after count and whereExists).

knex('people').exists(function() {
  this.select('*').from('order').where('person_id', this.ref('people.id');
}, { as: 'has_orders' });

Alternatively, it could take its alias_name as the first parameter, as window functions like row_number do.