[Improvement] upgrade to precss 3.0 (#1696)

packages/vant-css/src/mixins/border-retina.css

@@ -1,34 +0,0 @@
-@import '../common/var.css';
-
-$border-poses: top, right, bottom, left;
-
-@define-mixin border-retina $poses: $border-poses, $border-retina-color: $border-color {
-  content: '';
-  position: absolute;
-  top: 0;
-  left: 0;
-  width: 200%;
-  height: 200%;
-  transform: scale(.5);
-  transform-origin: 0 0;
-  pointer-events: none;
-  box-sizing: border-box;
-
-  @each $pos in $poses {
-    border-$(pos): 1px solid $border-retina-color;
-  }
-}
-
-@define-mixin hairline $border-retina-color: $border-color {
-  content: '';
-  position: absolute;
-  top: 0;
-  left: 0;
-  width: 200%;
-  height: 200%;
-  transform: scale(.5);
-  transform-origin: 0 0;
-  pointer-events: none;
-  box-sizing: border-box;
-  border: 0 solid $border-retina-color;
-}

packages/vant-css/src/mixins/clearfix.css

@@ -1,4 +1,4 @@
-@define-mixin clearfix {
+@mixin clearfix {
   &::after {
     content: '';
     display: table;

packages/vant-css/src/mixins/ellipsis.css

@@ -1,13 +1,13 @@
-@define-mixin multi-ellipsis $lines {
-  overflow : hidden;
+@mixin multi-ellipsis($lines) {
+  overflow: hidden;
   text-overflow: ellipsis;
   display: -webkit-box;
   -webkit-line-clamp: $lines;
   -webkit-box-orient: vertical;
 }
 
-@define-mixin ellipsis {
-    overflow: hidden;
-    white-space: nowrap;
-    text-overflow: ellipsis;
-}
+@mixin ellipsis {
+  overflow: hidden;
+  white-space: nowrap;
+  text-overflow: ellipsis;
+}

packages/vant-css/src/mixins/hairline.css

@@ -0,0 +1,13 @@
+@mixin hairline($border-color: #e5e5e5) {
+  content: '';
+  position: absolute;
+  top: 0;
+  left: 0;
+  width: 200%;
+  height: 200%;
+  transform: scale(0.5);
+  transform-origin: 0 0;
+  pointer-events: none;
+  box-sizing: border-box;
+  border: 0 solid $border-color;
+}