Ability to add dividers to the list of tabs within a window

This was a TO feature I used frequently, and I would like to see it in TabFern. A tree item, instead of being a link to a tab, can be a horizontal rule or other visual divider.

No files, tests, or entry points are named. Start by locating how the window's tab tree renders and stores tree items, then determine how a horizontal rule or visual divider should be represented and preserved; done means dividers appear among tabs without acting as links.

Written by the indexing model from the issue text.