Add stylish-haskell as a fixer (#1837)

* Add stylish-haskell as a fixer

`stylish-haskell` is a common formatting tool for the haskell toolchain.
It is not as advanced as `brittany` or `hindent`, but it is commonly
used for formatting of imports and data declarations. This adds it as a
fixer in ALE.

autoload/ale/fix/registry.vim

@@ -170,6 +170,11 @@ let s:default_registry = {
 \       'suggested_filetypes': ['haskell'],
 \       'description': 'Fix Haskell files with brittany.',
 \   },
+\   'stylish-haskell': {
+\       'function': 'ale#fixers#stylish_haskell#Fix',
+\       'suggested_filetypes': ['haskell'],
+\       'description': 'Refactor Haskell files with stylish-haskell.',
+\   },
 \   'refmt': {
 \       'function': 'ale#fixers#refmt#Fix',
 \       'suggested_filetypes': ['reason'],