Add new `reverse` value to `wildoptions`

I propose this new value for wildoptions which, together with pum, displays the popup menu matches in reverse order (the first match is at the bottom and <Tab> moves the selection upwards).

This is useful in combination with the snippets described in :h fuzzy-file-picker and :h live-grep where the command line is populated with many suggestions. It'd be better if the first suggestions were shown close to the command line prompt instead of at the top of the screen.