use new stylelint extension for VS Code

Created by: sqs

According to https://github.com/stylelint/stylelint/pull/4438, the shinnn.stylelint extension was deleted because its creator abandoned it. The fork at https://marketplace.visualstudio.com/items?itemName=hex-ci.stylelint-plus is still maintained.

Fixes an issue where VS Code reports that one of the recommended workspace extensions does not exist in the VS Code extension marketplace.

• Blocked on https://github.com/hex-ci/vscode-stylelint-plus/issues/8