Initial implementation of scrolling menu dropdown based on patch from...

https://www.drupal.org/project/admin_toolbar/issues/2917168

Initial implementation of scrolling menu dropdown based on patch from DO-2917168-55 and the work by @gallant_dev and @klonos.

Scrolling menu dropdown:
If the dropdown menu is too long vertically, it will reach a maximum height and display a scrollbar allowing scrolling inside the menu to display other menu items.