Add dialect-specific extension point to Expr

[jdegoes]

Currently, Expr is closed, and subtypes of the Sql module cannot add new expressions. This is inconvenient as different dialects of SQL support different constructs.

To support extensibility, we can add a new constructor for Expr that holds a module-defined type, e.g:

type DialectSpecificExpr[Features, -Input, Output]

...

object Expr {
  ...
  sealed case class DialectSpecific[F, -A, B](dialectSpecific: DialectSpecificExpr[F, A, B]) extends Expr[F, A, B]
}

This way, a dialect that has no extension can define type DialectSpecificExpr[F, I, O] = Nothing, while others can define it to be a custom type.

[jczuchnowski]

An initial approach was proposed in #309, but it had to be closed for now due to conflicts.