Popconfirm: Add popconfirm component (#17548)

* init

* init

* init docs

* init docs2

* init type

* update test

* update naming

test/unit/specs/popconfirm.spec.js

@@ -0,0 +1,29 @@
+import { createVue, destroyVM } from '../util';
+
+describe('Popconfirm', () => {
+  let vm;
+  afterEach(() => {
+    destroyVM(vm);
+  });
+
+  describe('trigger', () => {
+    const createVM = () => {
+      return createVue(`
+        <div>
+          <el-popconfirm
+            ref="popover"
+            title="content">
+            <button slot="reference">trigger</button>
+          </el-popconfirm>
+        </div>
+      `, true);
+    };
+    it('click', () => {
+      vm = createVM();
+      vm.$el.querySelector('button').click();
+      document.body.click();
+      expect(document.body.querySelector('.el-popconfirm__action').style.display).to.equal('');
+    });
+  });
+
+});