feat: migrate Toast component

src/toast/index.less

@@ -1,9 +1,6 @@
 @import '../style/var';
 
 .van-toast {
-  position: fixed;
-  top: 50%;
-  left: 50%;
   display: flex;
   flex-direction: column;
   align-items: center;
@@ -25,7 +22,6 @@
   word-wrap: break-word;
   background-color: @toast-background-color;
   border-radius: @toast-border-radius;
-  transform: translate3d(-50%, -50%, 0);
 
   &--unclickable {
     // lock scroll