Support Deeper Navigation Levels

Problem/Motivation

Initial implementation supports 3 levels but we want designs for 4 levels of menu hierarchy, since several contrib need them.

Steps to reproduce

1. Access admin/config/user-interface/navigation-block
2. Edit any existing menu with enough deep levels
3. Change the Number of levels to display to 4

Proposed resolution

Update the implementation with the 4th level designs to support another level of navigation.

Link to Figma designs.

Remaining tasks

Implement the new designs.