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

Since [version 13.6.0](https://github.com/stylelint/stylelint/blob/804bb24c75248aba55b009994e4bfb561593a990/CHANGELOG.md?plain=1#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/scss/stylelint.vim

@@ -11,6 +11,7 @@ endfunction
 
 call ale#linter#Define('scss', {
 \   'name': 'stylelint',
+\   'output_stream': 'both',
 \   'executable': {b -> ale#path#FindExecutable(b, 'scss_stylelint', [
 \       'node_modules/.bin/stylelint',
 \   ])},