Samuel Mešša

### Description [Expr.str.concat](https://docs.pola.rs/py-polars/html/reference/expressions/api/polars.Expr.str.concat.html#polars.Expr.str.concat) has a `delimiter` argument while [pl.concat_str](https://docs.pola.rs/py-polars/html/reference/expressions/api/polars.concat_str.html) has a `separator` argument. Conceptually they represent the same thing. Maybe this could be unified 🙏 ? Willing to do it...