[improvement] add slide-down class (#2134)

packages/style/animation.less

@@ -1,15 +1,27 @@
-@keyframes van-slide-bottom-enter {
+@keyframes van-slide-up-enter {
   from {
     transform: translate3d(0, 100%, 0);
   }
 }
 
-@keyframes van-slide-bottom-leave {
+@keyframes van-slide-up-leave {
   to {
     transform: translate3d(0, 100%, 0);
   }
 }
 
+@keyframes van-slide-down-enter {
+  from {
+    transform: translate3d(0, -100%, 0);
+  }
+}
+
+@keyframes van-slide-down-leave {
+  to {
+    transform: translate3d(0, -100%, 0);
+  }
+}
+
 @keyframes van-fade-in {
   from {
     opacity: 0;
@@ -50,12 +62,22 @@
   }
 }
 
-.van-slide-bottom {
+.van-slide-up {
   &-enter-active {
-    animation: van-slide-bottom-enter .3s both ease;
+    animation: van-slide-up-enter .3s both ease;
   }
 
   &-leave-active {
-    animation: van-slide-bottom-leave .3s both ease;
+    animation: van-slide-up-leave .3s both ease;
+  }
+}
+
+.van-slide-down {
+  &-enter-active {
+    animation: van-slide-down-enter .3s both ease;
+  }
+
+  &-leave-active {
+    animation: van-slide-down-leave .3s both ease;
   }
 }