Allow using StyleVariables in calculations

[bitspittle]

For example: 1.px + SomeLengthVar.value()

Right now this compiles but it breaks.

What it should do is convert into something like calc(1.px + --some-length()).

It should be noted that StyleVariable.value() works by doing a force cast, which is probably why this isn't working.

If we can get it to work directly as is above, that would be great. Back up plan would be to add operator fun overloading to variables directly: 1.px + SomeLengthVar and SomeLengthVar + 1.ps

[DennisTsar]

Fixed in 0.13.11. See: https://github.com/varabyte/kobweb#arithmetic-for-stylevariables-using-calc