feat(Fish): Set default editor to Neovim

Using the recommendation in the FAQ
(https://fishshell.com/docs/current/faq.html#why-doesn-t-set-ux-exported-universal-variables-seem-to-work),
set the `EDITOR` environment variable to `nvim`.

fish/config.fish

@@ -2,3 +2,5 @@ if status is-interactive
     # Commands to run in interactive sessions can go here
     [ -r $HOME/.sh.d/aliases ] && . $HOME/.sh.d/aliases
 end
+
+set -gx EDITOR nvim