Keep zero-length units inside all CSS math functions

[sarathfrancis90]

While minifying a stylesheet that uses round() I noticed the output had broken values. csso strips the unit off a zero length inside most math functions:

round(0px, 3px)  ->  round(0, 3px)
mod(0px, 3px)    ->  mod(0, 3px)
hypot(0px, 3px)  ->  hypot(0, 3px)
calc(1px + round(0px, 3px))  ->  calc(1px + round(0, 3px))

Inside a math function a unitless zero is a <number>, not a zero <length>, so after the rewrite the two arguments no longer share a type. The value is invalid and the whole declaration is dropped by the browser, so the styling silently disappears.

The zero-length optimization already guards against this for calc(), min(), max() and clamp() (issue #222, #426), but the MATH_FUNCTIONS set was never extended to the newer math functions. This fills in the rest of them (round, mod, rem, the trigonometric, exponential and sign-related functions) so their zero-length arguments keep the unit.

Tested with a new fixtures/compress/units/5 case; npm run lint and npm test are green.