feat(dockview-core): add slide tab reorder mode with FLIP animations

[KyDenZ]

Summary

Introduces smoothTabReorder: boolean — a new option that enables browser-tab-like slide animations during drag-and-drop tab reorder. Defaults to false (existing behavior unchanged).

Changes

During drag, tabs slide apart to open a gap showing where the dragged tab will land
On drop, tabs animate to their final positions
The dragged tab collapses in-place (dv-tab--dragging) so the gap is clearly visible
Respects prefers-reduced-motion media query

Docs

Added template with a toggle to enable/disable smoothTabReorder in tabview sandbox

[codesandbox-ci]

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Latest deployment of this branch, based on commit 1034e93:

Sandbox	Source
dockview-app	Configuration
editor-gridview	Configuration
externaldnd-dockview	Configuration
fullwidthtab-dockview	Configuration
iframe-dockview	Configuration
keyboard-dockview	Configuration
nativeapp-dockview	Configuration
rendering-dockview	Configuration

[mathuo]

@KyDenZ don't suppose you have a quick gif of the behavior?

[KyDenZ]

Sure! Here are two recordings showing the feature in action:

Default behavior: smoothTabReorder: false

New behavior: smoothTabReorder: true

Enabling smoothTabReorder makes tabs slide apart during drag to reveal the insertion gap, then animate to their final positions on drop. The default behavior is unchanged.