vim: move 'move' to rustStorage

It used to be in `rustKeyword`, until commit 5c75f210ba6e450fb1603b50ca0a4805f13173d7 removed it, and then #18782 restored it again.  However, this is now a closure modifier, and I think moving it to `rustStorage` is more appropriate to highlight it similarly to `mut`, `ref`, and the `&` sigil.