Add alt+a shortcut for search form and README

Closes #3494646

admin_toolbar_search/js/admin_toolbar_search.js

@@ -85,6 +85,14 @@
           $searchTab.toggleClass('visible');
           $searchInput.focus();
         });
+
+        // Shortcut 'alt + a' will focus on the search form.
+        document.addEventListener('keydown', (event) => {
+          if (event.altKey && event.key === 'a') {
+            context.getElementById('admin-toolbar-search-input').focus();
+          }
+        });
+
       });
     },
     getItemLabel: function (item) {