[improvement] Cell: optimize border (#1902)

packages/vant-css/src/cell.css

@@ -13,8 +13,11 @@
   overflow: hidden;
 
   &:not(:last-child)::after {
-    margin-left: 15px;
-    border-bottom-width: 1px;
+    @include hairline-bottom($border-color, 15px);
+  }
+
+  &--borderless::after {
+    display: none;
   }
 
   &-group {

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

@@ -1,12 +1,27 @@
-@mixin hairline($border-color: #eee) {
-  content: '';
+@mixin hairline-common() {
+  content: ' ';
   position: absolute;
+  pointer-events: none;
+  box-sizing: border-box;
+}
+
+@mixin hairline($border-color: #eee) {
+  @include hairline-common;
+
   top: -50%;
   left: -50%;
   right: -50%;
   bottom: -50%;
   transform: scale(0.5);
-  pointer-events: none;
-  box-sizing: border-box;
   border: 0 solid $border-color;
 }
+
+@mixin hairline-bottom($border-color: #eee, $left: 0) {
+  @include hairline-common;
+
+  left: $left;
+  right: 0;
+  bottom: 0;
+  transform: scaleY(0.5);
+  border-bottom: 1px solid $border-color;
+}