Feature: #107716 - Third-level modules in backend module menu 

Description 

The backend module menu now supports display of three levels of module nesting, improving navigation for complex module structures.

Previously, the module menu only displayed two levels: main modules and their direct submodules. With this feature, submodules can now have their own child modules (third level), which are displayed in the menu with proper visual hierarchy.

Third-level modules are displayed without icons and with increased indentation to distinguish them from their parent modules. Users can expand or collapse second-level modules that have third-level children by clicking on the chevron indicator next to the module name.

The module link itself remains clickable to navigate to the module, while the indicator toggles the visibility of child modules.

The new level is also fully accessible via keyboard.

Impact 

Extension developers can now create deeper module hierarchies with up to three levels of nesting, allowing for better organization of complex module structures. The expand/collapse functionality is fully keyboard accessible and provides a cleaner, more organized navigation experience.