Add dedicated $dropdown-divider-margin-y variable

Created by: spaceemotion

This is a feature request.

Currently the .dropdown-divider class uses the general nav-divider()-mixin with the default $nav-divider-margin-y vertical margin.

If I set my dropdown vertical padding (e.g. $dropdown-padding-y: 0.25rem), there is no way to only change the vertical margin of the divider without affecting other navigation elements currently.

It would be nice to have a variable like $dropdown-divider-margin-y to set this specifically. I can open a PR for this, if this is to be added.