Fix prefixing for grid-column/row-span

[etripier]

It turns out that using an integer value for msGrid(Column|Row)Span causes the style to be interpreted as a unitless length value, which means it ends up as ${value}px instead of just ${value}. Unfortunately, that's not what we want - we expect this to refer to a line position on the grid.

I also took the opportunity to fix #176, although I think the conditions for producing that error are far off the beaten path.

[etripier]

@robinweser @lencioni Hello there! Would you please review this when you have time?

[lencioni]

I'm confused why ms is a special case for this package. I'm wondering if the root problem you are seeing is somewhere else?

[robinweser]

@etripier Hmm. Seems more like an issue with the underlying styling library?