Closes https://github.com/zed-industries/zed/issues/32181,
https://github.com/zed-industries/zed/issues/32179

In the screenshots: left = nightly, right = dev

Fix 1: A pinned tab dragged into a new split should remain pinned


https://github.com/user-attachments/assets/608a7e10-4ccb-4219-ba81-624298c960b0

Fix 2: Moving a pinned tab from one pane to another should not cause
other pinned tabs to be unpinned


https://github.com/user-attachments/assets/ccc05913-591d-4a43-85bb-3a7164a4d6a8

I also added tests for moving both pinned tabs and unpinned tabs into
existing panes, both into the "pinned" region and the "unpinned" region.

Release Notes:

- Fixed a bug where dragging a pinned tab into a new split would lose
its pinned tab state.
- Fixed a bug where pinned tabs in one pane could be lost when moving
one of the pinned tabs to another pane.