Add luafmt fixer (#3289)

autoload/ale/fix/registry.vim

@@ -375,6 +375,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['html', 'htmldjango'],
 \       'description': 'Fix HTML files with html-beautify.',
 \   },
+\   'luafmt': {
+\       'function': 'ale#fixers#luafmt#Fix',
+\       'suggested_filetypes': ['lua'],
+\       'description': 'Fix Lua files with luafmt.',
+\   },
 \   'dhall': {
 \       'function': 'ale#fixers#dhall#Fix',
 \       'suggested_filetypes': ['dhall'],