feat(Popover): add default slot

src/popover/index.less

@@ -4,7 +4,7 @@
 .van-popover {
   position: absolute;
   overflow: visible;
-  border-radius: @border-radius-lg;
+  background-color: transparent;
   transition: opacity 0.15s, transform 0.15s;
 
   &__wrapper {
@@ -20,6 +20,11 @@
     border-width: 6px;
   }
 
+  &__content {
+    overflow: hidden;
+    border-radius: @border-radius-lg;
+  }
+
   &__action {
     position: relative;
     display: flex;
@@ -191,8 +196,11 @@
 
   &--light {
     color: @text-color;
-    background-color: @white;
-    box-shadow: 0 2px 12px rgba(50, 50, 51, 0.12);
+
+    .van-popover__content {
+      background-color: @white;
+      box-shadow: 0 2px 12px rgba(50, 50, 51, 0.12);
+    }
 
     .van-popover__arrow {
       color: @white;
@@ -215,7 +223,10 @@
 
   &--dark {
     color: @white;
-    background-color: #4a4a4a;
+
+    .van-popover__content {
+      background-color: #4a4a4a;
+    }
 
     .van-popover__arrow {
       color: #4a4a4a;