fix(mover): Don't heal rows when disconnecting at start of move

Modify KeyboardDragStrategy.prototype.shouldHealStack so that
only stacks will be healed.

Author: cpcallen

src/keyboard_drag_strategy.ts

@@ -326,6 +326,6 @@ export class KeyboardDragStrategy extends dragging.BlockDragStrategy {
   }
 
   override shouldHealStack(e: PointerEvent | undefined): boolean {
-    return true;
+    return Boolean(this.block.previousConnection);
   }
 }