Stylelint linter output stream can go to stderr (#4702)

Since [version 13.6.0](804bb24c75/CHANGELOG.md (L654)),
following [PR 4799](https://github.com/stylelint/stylelint/pull/4799)
`stylelint` errors are sent to `stderr`. Previous versions where sending
errors to `stdout`.

ale_linters/sugarss/stylelint.vim

@@ -13,6 +13,7 @@ endfunction
 
 call ale#linter#Define('sugarss', {
 \   'name': 'stylelint',
+\   'output_stream': 'both',
 \   'executable': {b -> ale#path#FindExecutable(b, 'sugarss_stylelint', [
 \       'node_modules/.bin/stylelint',
 \   ])},