Fix a bug where VS Code's eslint integration does not work for the newtab project because of a conflict caused by sass-lint's eslint dependency. Switching over to stylelint eliminates that dependency and generally modernizes our SCSS linting. stylelint doesn't have a 1 to 1 replacement for every sass-lint rule, so a few rules have been changed. Differential Revision: https://phabricator.services.mozilla.com/D173605