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GH-36414: [C++] Add missing type_traits.h predicate: is_var_length_list()  (#36415)

### Rationale for this change

`is_var_length_list_type<T>` is missing its `is_var_length_list(type_id)` counterpart.

### What changes are included in this PR?

 - New function
 - Use of `is_var_length_list_type<T>` directly (instead of using deprecated `is_base_list_type<T>`

### Are these changes tested?

Yes.
* Closes: #36414

Lead-authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Antoine Pitrou <antoine@python.org>