feat(Popover): improve a11y (#7624)

src/popover/index.js

@@ -114,13 +114,14 @@ export default createComponent({
     renderAction(action, index) {
       const { icon, text, disabled, className } = action;
       return (
-        <div
+        <button
+          type="button"
           class={[bem('action', { disabled, 'with-icon': icon }), className]}
           onClick={() => this.onClickAction(action, index)}
         >
           {icon && <Icon name={icon} class={bem('action-icon')} />}
           <div class={[bem('action-text'), BORDER_BOTTOM]}>{text}</div>
-        </div>
+        </button>
       );
     },
 

src/popover/index.less

@@ -34,6 +34,8 @@
     padding: 0 @padding-md;
     font-size: @popover-action-font-size;
     line-height: @line-height-md;
+    background-color: transparent;
+    border: none;
     cursor: pointer;
 
     &:last-child {

src/popover/test/__snapshots__/index.spec.js.snap

@@ -1,9 +1,9 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
 exports[`should allow to custom the className of action 1`] = `
-<div class="van-popover__action foo">
+<button type="button" class="van-popover__action foo">
   <div class="van-popover__action-text van-hairline--bottom">Option</div>
-</div>
+</button>
 `;
 
 exports[`should locate to reference element when showed 1`] = `<!---->`;