`items` prop of SortableContext should be ReadonlyArray

[wand2016]

https://github.com/clauderic/dnd-kit/blob/694dcc2f62e5269541fc941fa6c9af46ccd682ad/packages/sortable/src/components/SortableContext.tsx#L11

In order to pass immutable array data, items prop should be:

ReadonlyArray<UniqueIdentifier | {
  id: UniqueIdentifier;
}>;