nixpkgs/neovim: support elvish in plenary.filetype

Among other things, this allows it to be properly highlighted in
telescope.

.config/nixpkgs/neovim/plugins.lua

@@ -1,3 +1,10 @@
+-- add extra filetypes for plenary
+require('plenary.filetype').add_table({
+  extension = {
+    ['elv'] = [[elvish]]
+  }
+})
+
 -- file/buffer/etc picker
 require('telescope').setup({
   defaults = {