Make active tab behaviour independent of html structure

Created by: alberto

In v4, the active tab selector is defined as:

ACTIVE_CHILD: '> .nav-item > .active, > .active',

The first selector forces us to use the nav-item class for the tab, which I would like to avoid in my case.

The second selector, which applies to the tab, not the link, doesn't seem to work correctly (see https://github.com/twbs/bootstrap/issues/18566)

Is the descendant selector really needed?