Split CSS vars for padding values (#35921)

Split CSS vars for padding values (#35921)

* Split CSS vars for padding values

Make these few components consistent with where we're heading with other components. Had to add some new Sass variables to handle the dropdown-header element, but not a huge deal. This ensures we can drop the combined variable in v6 when we're ready.

* Update scss/_dropdown.scss

Co-authored-by: Julien Déramond <julien.deramond@orange.com>
* Deprecate dropdown-header-padding var

Co-authored-by: Gaël Poupard <ffoodd@users.noreply.github.com>
Co-authored-by: Julien Déramond <julien.deramond@orange.com>