ToolbarHandler should be a service

Implemented ToolbarHandler as a service. Please check.

This looks good to me, a simple but effective change.

I checked the commit and that is exactly what the reporter asked for and what I was about to do.

Applied merge request !41 as a patch to branch 4.X.

Tested on a Drupal 9.5.7 and php 8.1.28 install and works fine. (see caption)

Thank you @Denist3r for your work!

Tested on a Drupal 10.2.6 and php 8.1.27 install and works fine. ( see caption → )

I've included new changes from 4.x, there was a conflict, and also formatted the service argument as a list. Thanks!