feat(plugin-search): improve a11y support (#165)

Author: Mister-Hope

packages/@vuepress/plugin-search/src/client/components/SearchBox.ts

@@ -94,13 +94,15 @@ export const SearchBox = defineComponent({
 
     return () =>
       h(
-        'div',
+        'form',
         {
           class: 'search-box',
+          role: 'search',
         },
         [
           h('input', {
             ref: input,
+            type: 'search',
             placeholder: locale.value.placeholder,
             autocomplete: 'off',
             spellcheck: false,