message: [improvement] Checkbox: use flex-box to align
diff: @import '../style/var';
  .van-checkbox {
  + display: flex;
  + align-items: center;
  overflow: hidden;
  user-select: none;
  - &__icon,
  - &__label {
  - display: inline-block;
  - vertical-align: middle;
  - }
  -
  &__icon {
  + flex: none;
  height: 1em;
  font-size: @checkbox-size;
  line-height: 1em;
  