Add stylish-haskell config.

I have used this one "successfully" in new modules for three
or so years.

.stylish-haskell.yaml

+steps:
+  - imports:
+      align: group
+      list_align: after_alias
+      long_list_align: new_line
+      empty_list_align: right_after
+      list_padding: module_name
+  - language_pragmas:
+      style: vertical
+      remove_redundant: true
+  - trailing_whitespace: {}
+columns: 160
+language_extensions:
+  - DataKinds
+  - DerivingStrategies
+  - ExplicitForAll
+  - FlexibleContexts
+  - GeneralizedNewtypeDeriving
+  - MultiParamTypeClasses