Add Regex as a valid column type in typespec

Dialyzer is complaining when we pass a regular expression to functions like `Explorer.DataFrame.select/2`, even though the documentation says it's a valid argument.

This commit solves the issue by adding `Regex.t()` to the type definition of `columns()` type.

lib/explorer/data_frame.ex

@@ -239,6 +239,7 @@ defmodule Explorer.DataFrame do
   @type columns ::
           [column]
           | Range.t()
+          | Regex.t()
           | (String.t() -> boolean())
           | (String.t(), Explorer.Series.dtype() -> boolean())