Move Nix files into a nix directory

Move everything from `config` to the root level.

nvim/after/ftplugin/gitcommit.lua

@@ -0,0 +1,8 @@
+vim.opt_local.colorcolumn = "50,72"
+vim.opt_local.spell = true
+vim.opt_local.textwidth = 72
+
+-- autocmd FileType gitcommit highlight ColorColumn ctermbg=8
+-- filetype indent on
+-- filetype on
+-- filetype plugin on