Positional arguments should work in @select and @rename

[davidanthoff]

We should support positional arguments in @select and @rename. For example

source |> @select(2, 5)

should select the second and fifth column.

I think in general positional args should also work with the - syntax, and with the range syntax.

I think for @rename we could also support them on the left side of the =>, right? That would mean "rename the xth column".