If result of evaluated javascript is a number return it as Dimension.

Fix for issue #1768

Use-case:
```
@outerWidthPx: 100px;
@borderWidthPx: 1px;

.element {
  width: @outerWidthPx - @borderWidthPx * 2;
  border: @borderWidthPx solid #000;

  &.small {
    // Half of @borderWidthPx, but not smaller than 1px
    @halfBorderWidth: unit(~`Math.max(1, Math.floor(parseInt("@{borderWidthPx}") / 2))`, px);

    border-width: @halfBorderWidth;
    width: floor(@outerWidthPx / 2) - @halfBorderWidth * 2;
  }
}
```