Update the implementation of the box-shadow #87

The proposal is to update the implementation of the box-shadow CSS property and make it close/similar to how it is currently implemented in Clay.
But this might be a breaking change, otherwise, I just don't see how to keep naming clear and simple.
In Clay they've just renamed the old/deprecated boxShadow to boxShadow' etc: https://github.com/sebastiaanvisser/clay/blob/54dc9eaf0abd180ef9e35d97313062d99a02ee75/src/Clay/Box.hs#L142, but I'm not sure if it makes sense to do the same thing here.