message: fix: missing var in hairline.less
diff: +@import './var';
  @import './mixins/hairline';
  [class*='van-hairline'] {
  }
  &--top::after {
  - border-top-width: 1px;
  + border-top-width: @border-width-base;
  }
  &--left::after {
  - border-left-width: 1px;
  + border-left-width: @border-width-base;
  }
  &--right::after {
  - border-right-width: 1px;
  + border-right-width: @border-width-base;
  }
  &--bottom::after {
  - border-bottom-width: 1px;
  + border-bottom-width: @border-width-base;
  }
  &,
  &-unset {
  &--top-bottom::after {
  - border-width: 1px 0;
  + border-width: @border-width-base 0;
  }
  }
  