Use new GOV.UK Frontend link styles

GOV.UK Frontend v3.12.0 comes with new (optional) link styles that are
designed to be easier to read and to have a clearer hover state [[1]].

This commit opts-in to the new link styles for the tech-docs-gem
stylesheet, and refactors the table of contents navigation and search
results links to use new the new link style mixins [[2]] so that they
are styled consistently with other links.

Note that this commit makes a small design change; search result links
are now underlined when not hovered.

[[1]]: https://github.com/alphagov/govuk-frontend/blob/main/CHANGELOG.md#links-are-easier-to-read-and-have-a-clearer-hover-state-opt-in
[[2]]: https://frontend.design-system.service.gov.uk/sass-api-reference/#helpers-links

lib/assets/stylesheets/_govuk_tech_docs.scss

@@ -1,6 +1,7 @@
 $govuk-assets-path: "/assets/govuk/assets/";
 
 // Include only the bits of GOV.UK Frontend we need
+$govuk-new-link-styles: true;
 @import "govuk/settings/all";
 @import "govuk/tools/all";
 @import "govuk/helpers/all";

lib/assets/stylesheets/modules/_search.scss

@@ -87,21 +87,9 @@ html.has-search-results-open {
       min-height: auto;
     }
 
-    a:link, a:visited {
-      text-decoration: none;
-      color: $govuk-link-colour;
-    }
-
-    a:hover {
-      text-decoration: underline;
-
-      &:focus {
-        text-decoration: none;
-      }
-    }
-    a:focus {
-      outline: none;
-      @include govuk-focused-text;
+    a {
+      @include govuk-link-common;
+      @include govuk-link-style-no-visited-state;
     }
   }
 .search-results__inner {

lib/assets/stylesheets/modules/_toc.scss

@@ -15,14 +15,18 @@
 
     &__list {
       li {
+        a {
+          @include govuk-link-decoration;
+          @include govuk-link-style-no-visited-state;
+          @include govuk-link-style-no-underline;
+        }
+
         a:link, a:visited {
           display: block;
           position: relative;
           padding: 8px govuk-spacing(6) 8px govuk-spacing(2);
           margin: 0 -1 * govuk-spacing(3);
           border-left: 5px solid transparent;
-          text-decoration: none;
-          color: $govuk-link-colour;
 
           @include govuk-media-query(tablet) {
             &.toc-link--in-view  {
@@ -33,21 +37,17 @@
           }
         }
 
+        a:hover {
+          @include govuk-link-hover-decoration;
+        }
+
         a:focus {
-          outline: none;
+          text-decoration: none;
 
           span {
             @include govuk-focused-text;
           }
         }
-
-        a:hover {
-          text-decoration: underline;
-
-          &:focus {
-            text-decoration: none;
-          }
-        }
       }
 
       // Top level