[new feature] add Collapse component (#674)

packages/vant-css/src/collapse.css

@@ -0,0 +1,31 @@
+@import './common/var.css';
+
+.van-collapse-item {
+  &__title {
+    .van-cell__right-icon::before {
+      transition: .3s;
+      transform: rotate(90deg);
+    }
+
+    &::after {
+      visibility: hidden;
+    }
+  }
+
+  &__content {
+    padding: 15px;
+    background-color: #fff;
+  }
+
+  &--expanded {
+    .van-collapse-item__title {
+      .van-cell__right-icon::before {
+        transform: rotate(-90deg);
+      }
+
+      &::after {
+        visibility: visible;
+      }
+    }
+  }
+}

packages/vant-css/src/index.css

@@ -13,6 +13,7 @@
 @import './button.css';
 @import './cell.css';
 @import './circle.css';
+@import './collapse.css';
 @import './loading.css';
 @import './nav-bar.css';
 @import './notice-bar.css';