add support for downcase-word, upcase-word and capitalise-word

Some muscle-memory pleasing for those with prolonged exposure to readline command line editing:

* `upcase-word` (`alt`+`U`)
* `downcase-word` (`alt`+`L`)
* `capitalise-word` (`alt`+`C`)

The keybinding for `downcase-word` clashes with the existing keybinding for `__fish_list_current_token`, so I'm leaving this disabled in the default keybindings.