Improve tab drag-and-drop between split panes

[RA1NCS]

Tab drag-and-drop across split panes should behave more like VS Code. Right now, moving tabs between panes feels limited and unintuitive. It should be easy to reorganize tabs without manually closing splits or reopening files.

Requested behavior:

1. Drag a tab from one split pane to another

   • A tab should be draggable from the right pane to the left pane, or from the left pane to the right pane.
   • The tab should be removed from its original pane and added to the target pane.

2. Close empty split panes automatically

   • If a split pane has only one tab and that tab is dragged into another pane, the now-empty pane should close.
   • The remaining pane should expand to fill the space.

3. Move one tab out of a multi-tab pane

   • If the right pane has two tabs and I drag one tab to the left pane, only that tab should move.
   • The right pane should stay open with the remaining tab.

4. Allow tab reordering within the same pane

   • Users should be able to drag tabs left or right within the same tab bar to reorder them.

5. Allow dropping into specific positions

   • When dragging a tab into another pane, the drop indicator should show where the tab will land.
   • Example: before the first tab, between tabs, or after the last tab.

6. Allow creating a new split by dragging

   • Dragging a tab to the left, right, top, or bottom edge of the editor should create a new split pane in that direction.

7. Preserve tab state after moving

   • Moving a tab should preserve scroll position, cursor position, unsaved edits, file state, and preview/editor state.

Expected result:

Tab management should feel fluid and predictable:

• Drag tab into another pane, it moves there.
• Drag the last tab out of a pane, the empty pane closes.
• Drag to an edge, a new split is created.
• Drag inside the same pane, tabs reorder.
• No file state should reset during the move.

Why this matters:

Split panes are useful only when tabs are easy to reorganize. VS Code gets this right, and Athas should support the same basic drag-and-drop behavior so users can manage files naturally without fighting the layout.

[RA1NCS]

Fixed in #670