Upgrade stylelint

[kamilzyla]

Problem

Currently rhino::lint_sass() uses stylelint v14 which is the last major version of supporting stylistic rules. With v15 stylelintdecided to delegate all formatting to pretty printers (e.g. prettier). It no longer cares about code formatting (whitespace, line breaks, etc.).

We don't want to stay with stylelint v14 forever, but plain upgrade to the latest version would remove a large portion rhino::lint_sass() functionality.

Solutions

After upgrading stylelint to latest version we'll need to make up for the lost functionality. Two approaches come to mind:

1. Use @stylistic/stylelint-plugin, which should essentially bring back the functionality.
2. Introduce a pretty printer (e.g. prettier) and integrate it into Rhino. Our API might need adjustment, e.g. rhino::format_sass() instead of rhino::lint_sass(fix = TRUE).