git: Change editor from vim to neovim

Neovim has some nice additions like honoring the blinking cursor from
kitty when in insert mode. I don't remember why I used vim instead of
neovim here, but neovim is mature enough that it should be an excellent
choice to use for many years to come.

git/.gitconfig

@@ -6,7 +6,7 @@
     gpgsign = true
 
 [core]
-    editor = vim
+    editor = nvim
     autocrlf = false
     pager = diff-so-fancy | less --tabs=4 -RFX
     excludesfile = ~/.gitignore