Alias-aware expr() support

[parisholley]

Is your feature request related to a problem? Please describe.

When using expr, unless you have a single table find() or a query builder with an alias, you can hit ambiguous column errors due to the lack of column prefixing. Eg: self-referential eager joins

Describe the solution you'd like
Expand expr to support passing a function like so:

expr((alias) => `lower(${alias}.column)`)

alternatively, perhaps we can add the ability to define the alias within find methods

[B4nan]

How do you expect that to work? expr is just an identity function, it needs to return string as it will be used as an object key. It also does not have any context so it would have to use global one?

Maybe we could instead have some known substring we would replace, maybe something like [::alias::] or even ${alias} as an escaped string. In the end we could use it automatically if we see a function in the argument.

[parisholley]

substring would be sufficient