message: fix(NavBar): icon align center
diff: .van-icon {
  color: @nav-bar-icon-color;
  - vertical-align: middle;
  }
  &__arrow {
  &__left,
  &__right {
  position: absolute;
  + top: 0;
  bottom: 0;
  + display: flex;
  + align-items: center;
  font-size: @font-size-md;
  cursor: pointer;
  }
  }
  &__text {
  - display: inline-block;
  margin: 0 -@padding-md;
  padding: 0 @padding-md;
  color: @nav-bar-text-color;
  - vertical-align: middle;
  &:active {
  background-color: @active-color;
  