add stylelint to core, fixes #4965

.editorconfig

.gitlab-ci.yml

.stylelintrc.json

package-lock.json

package.json

resources/assets/stylesheets/fullcalendar.scss

resources/assets/stylesheets/highcontrast.scss

resources/assets/stylesheets/mixins/arrow.scss

resources/assets/stylesheets/mixins/colors.scss

resources/assets/stylesheets/mixins/misc.scss

resources/assets/stylesheets/mixins/studip.scss

resources/assets/stylesheets/scss/actionmenu.scss

resources/assets/stylesheets/scss/admin.scss

resources/assets/stylesheets/scss/ajax.scss

resources/assets/stylesheets/scss/avatar.scss

resources/assets/stylesheets/scss/blubber.scss

resources/assets/stylesheets/scss/breakpoints.scss

resources/assets/stylesheets/scss/buttons.scss

resources/assets/stylesheets/scss/calendar.scss

resources/assets/stylesheets/scss/courseware.scss