Add many math functions

[vrugtehagel]

This PR adds all functions from #205 except round(), since that one specifically accepts a keyword for its first argument. Probably I'll handle that one at some point in the future.

I've opted to restructure min, max and clamp slightly here to reduce on repetition. These CSS functions differ only really in their return value; the system already didn't check for the units inside calculations, e.g. you'd never have an angle in a width: calculation, but width: calc(10deg) highlights just fine.

However caring about the return value means that e.g. width: sin(10deg) is not highlighted (since sin() returns a number, not a length) but line-height: sin(10deg) does highlight. min(), max() and clamp() fall in the bucket of functions that can return anything, and their return value depends on their arguments.

[argyleink]

rad! these functions are so great, thanks for submitting highlighting enhancements for them and studying their types/syntax