Add highlighter styles

Author: frankieroberto

app/styles/application.scss

@@ -2,6 +2,9 @@
 
 @forward "pkg:@x-govuk/nhsuk-eleventy-plugin";
 
+// Add the highlighter styles
+@forward "pkg:nhsuk-frontend/lib/highlighter/styles";
+
 // stylelint-disable-next-line scss/load-no-partial-leading-underscore
 @forward "../_components";