Add valid css units

[atusy]

This PR adds valid css units such as ch and rem.

I'd also want to support calc().
However, regex for fully validating it will be quite complex.
What about just add calc(.*) ?

[jcheng5]

calc(.*) is totally fine IMHO, or I guess ^calc\\(.*\\)$

Thanks for the contribution! Have you ever filed a contributor agreement with RStudio before? If not, we'll need that on file before we can merge. Sorry for the inconvenience.

Thanks again!

[atusy]

Thanks. I added calc\\(.*\\)

About the contributor agreement, yes, I have when I contributed to bookdown.