Initial implementation of the 'toggle_shortcut' feature, based on MR 70.

https://www.drupal.org/project/admin_toolbar/issues/3304810

Initial implementation of enable_toggle_shortcut configuration, based on the work from MR !70 by @sascha_meissner @Grevil and @thomas.frobieter.

• Added configuration setting enable_toggle_shortcut (with install and schema) to admin_toolbar as a checkbox field enabled by default, in the Advanced section of the settings form.
• Updated module's logic to load added toggle_shortcut library based on configuration.
• Updated CSS and JS logic based on existing work for the sticky_behavior (MR !123 (merged)).
• Added basic Functional Tests.

config/install/admin_toolbar.settings.yml

+enable_toggle_shortcut: false
 menu_depth: 4
 sticky_behavior: 'enabled'
 hoverintent_behavior: