style: add safe-area mixin (#8062)

src/submit-bar/index.less

@@ -1,4 +1,5 @@
 @import '../style/var';
+@import '../style/mixins/safe-area.less';
 
 .van-submit-bar {
   position: fixed;
@@ -6,10 +7,9 @@
   left: 0;
   z-index: @submit-bar-z-index;
   width: 100%;
-  padding-bottom: constant(safe-area-inset-bottom);
-  padding-bottom: env(safe-area-inset-bottom);
   background-color: @submit-bar-background-color;
   user-select: none;
+  .safe-area-inset-bottom();
 
   &__tip {
     padding: @submit-bar-tip-padding;