Returning to support expressions and queries

[martijndeh]

The .returning() function currently only supports strings (which should be the target table's column names). In reality they can accept any expression and (returning) query.

This should be supported:

insert into bar (val) values (123) returning (select count(*) from bar), id, val || 'test'

which would equate to something like:

db.insertInto(bar).values({ val: 123 }).returning(db.select(count()).from(bar), concat(bar.val, `test`).as(`test`))

The returning function is used in insert, update and delete queries. So probably best to focus on getting one of those working. And later maybe create some reusable type if that's at all possible similar to something like SelectFn.

The return type in ResultSet<Returning, Test> probably already supports getting the type right.