refactor: pass lighthouse SEO audits (#261)

* fix(header): add aria-role for theme-toggle

GoogleChrome/lighthouse#12030 (comment)

* refactor: expand switch role to sr-only elements

switch is more meaningful than button

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a#technical_summary

layouts/partials/header.html

@@ -25,14 +25,14 @@
             <span class="nav-icons-divider"></span>
             <div class="nav-link dark-theme-toggle">
                 <span class="sr-only dark-theme-toggle-screen-reader-target"></span>
-                <a>
+                <a role="switch">
                     <span class="theme-toggle-icon" data-feather="moon"></span>
                 </a>
             </div>
 
             <div class="nav-link" id="hamburger-menu-toggle">
                 <span class="sr-only hamburger-menu-toggle-screen-reader-target">menu</span>
-                <a>
+                <a role="switch">
                     <span data-feather="menu"></span>
                 </a>
             </div>
@@ -48,7 +48,7 @@
                 {{ end }}
                 <li class="nav-item dark-theme-toggle">
                     <span class="sr-only dark-theme-toggle-screen-reader-target">theme</span>
-                    <a>
+                    <a role="switch">
                         <span class="theme-toggle-icon" data-feather="moon"></span>
                     </a>
                 </li>