If you were willing to give up a bit of magic, you could probably build this as a thin layer over PgTyped.

The API could be something like this:

Query.sql

  SELECT * FROM user LEFT JOIN post ON user.id = post.id

Application.ts

  const results = await Query()
  const nested = nest(results, {
    parentFields: ['userId', 'name'],
    childFields: ['postId', 'title'],
    childName: 'posts'
  )

If you wanted, you wouldn't really have to specify child fields, since they'd just whatever wasn't a parent field. It'd take a bit more work to get it to do multiple levels of nesting, but after a point it doesn't make sense to write queries that return so much duplicate data anyway.