message: [improvement] Loading: remove z-index
diff: .van-loading {
  position: relative;
  - z-index: 0;
  font-size: 0;
  vertical-align: middle;
  &__spinner {
  position: relative;
  - z-index: -1;
  display: inline-block;
  width: 30px;
  // compatible for 1.x, users may set width or height in root element
  align-items: center;
  .van-loading__text {
  - margin-top: 10px;
  - margin-left: 0;
  + margin: 10px 0 0;
  }
  }
  }
  