fix(theme-default): show header anchors when being focused (#164)

Author: Mister-Hope

Diff for: packages/@vuepress/theme-default/src/client/styles/_normalize.scss

@@ -107,6 +107,10 @@ a.header-anchor {
   &:hover {
     text-decoration: none;
   }
+
+  &:focus-visible {
+    opacity: 1;
+  }
 }
 
 code,