vim: Support updating dein (#857)

`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.

src/steps/upgrade.vim

@@ -23,6 +23,11 @@ if exists(":PackerUpdate")
     PackerSync
 endif
 
+if exists("*dein#update()")
+    echo "dein#update()"
+    call dein#update()
+endif
+
 if exists(":DeinUpdate")
     echo "DeinUpdate"
     DeinUpdate