`dein` is a plugin manager for vim, which was supported for a while in
an earlier version of topgrade. This patch reintroduces support for
upgrading "plain" `dein`, rather that the `dein` UI which is invoked by
`DeinUpdate` in vim.
The project https://github.com/wsdjeg/dein-ui.vim provides a userfriendly alternative to the `call dein#upgrade` call of the dein plugin manager. If this command is available, we should run it
