Add commentstring for nix file format (#12696)

runtime/ftplugin/nix.vim

@@ -0,0 +1,17 @@
+" Vim filetype plugin
+" Language:    nix
+" Maintainer:  Keith Smiley <keithbsmiley@gmail.com>
+" Last Change: 2023 Jul 22
+
+" Only do this when not done yet for this buffer
+if exists("b:did_ftplugin")
+  finish
+endif
+
+" Don't load another plugin for this buffer
+let b:did_ftplugin = 1
+
+let b:undo_ftplugin = "setl commentstring< comments<"
+
+setlocal comments=:#
+setlocal commentstring=#\ %s