Add nixpkgs-fmt fixer.

This commit adds a fixer for the Nix language, nixpkgs-fmt
(https://github.com/nix-community/nixpkgs-fmt).

autoload/ale/fix/registry.vim

@@ -335,6 +335,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['ada'],
 \       'description': 'Format Ada files with gnatpp.',
 \   },
+\   'nixpkgs-fmt': {
+\       'function': 'ale#fixers#nixpkgsfmt#Fix',
+\       'suggested_filetypes': ['nix'],
+\       'description': 'A formatter for Nix code',
+\   },
 \}
 
 " Reset the function registry to the default entries.