A more customizable tunnel feature

We (mostly I) need a more customizable tunnel feature. This includes:

- tunneling in parametrizable dimensions (.b tunnel with*height);
- diagonal tunneling

I heard that there are logistical problems to extend the current tunnel feature for these options, but maybe it could become a separate module.

The issue names no files, tests, or entry points, so first locate the current tunnel feature and understand its existing dimensions and command behavior. Done means supporting parameterized tunnel dimensions and diagonal tunneling, with the intended behavior and module boundaries defined clearly enough to implement and verify.

Written by the indexing model from the issue text.