fix: suppress stylelint false positive for notification banner scope extends

GOV.UK Frontend exposes no placeholder selectors for notification banner
heading or link, so class selectors are the only option.

Author: alexluckett

src/client/stylesheets/_prose.scss

@@ -61,12 +61,15 @@
   }
 }
 
+// GOV.UK Frontend does not expose placeholder selectors for these — class selectors are the only option.
 .app-prose-notification-banner-scope {
   p {
+    // stylelint-disable-next-line scss/at-extend-no-missing-placeholder
     @extend .govuk-notification-banner__heading;
   }
 
   a {
+    // stylelint-disable-next-line scss/at-extend-no-missing-placeholder
     @extend .govuk-notification-banner__link;
   }
 }