[improvement] add animation-duration less vars (#3916)

src/style/animation.less

@@ -1,3 +1,5 @@
+@import "./var";
+
 @keyframes van-slide-up-enter {
   from {
     transform: translate3d(0, 100%, 0);
@@ -78,50 +80,50 @@
 
 .van-fade {
   &-enter-active {
-    animation: .3s van-fade-in;
+    animation: @animation-duration-base van-fade-in;
   }
 
   &-leave-active {
-    animation: .3s van-fade-out;
+    animation: @animation-duration-base van-fade-out;
   }
 }
 
 .van-slide-up {
   &-enter-active {
-    animation: van-slide-up-enter .3s both ease;
+    animation: van-slide-up-enter @animation-duration-base both ease;
   }
 
   &-leave-active {
-    animation: van-slide-up-leave .3s both ease;
+    animation: van-slide-up-leave @animation-duration-base both ease;
   }
 }
 
 .van-slide-down {
   &-enter-active {
-    animation: van-slide-down-enter .3s both ease;
+    animation: van-slide-down-enter @animation-duration-base both ease;
   }
 
   &-leave-active {
-    animation: van-slide-down-leave .3s both ease;
+    animation: van-slide-down-leave @animation-duration-base both ease;
   }
 }
 
 .van-slide-left {
   &-enter-active {
-    animation: van-slide-left-enter .3s both ease;
+    animation: van-slide-left-enter @animation-duration-base both ease;
   }
 
   &-leave-active {
-    animation: van-slide-left-leave .3s both ease;
+    animation: van-slide-left-leave @animation-duration-base both ease;
   }
 }
 
 .van-slide-right {
   &-enter-active {
-    animation: van-slide-right-enter .3s both ease;
+    animation: van-slide-right-enter @animation-duration-base both ease;
   }
 
   &-leave-active {
-    animation: van-slide-right-leave .3s both ease;
+    animation: van-slide-right-leave @animation-duration-base both ease;
   }
 }