Only deg is supported as an angular unit for hues

[Crissov]

Currently, src/util.js has a TODO note to handle more (angular) units. These are rad, gradandturn` in CSS.

I believe it would also be useful to accept the degree sign ° U+00B0 (i.e. outside US-ASCII) as a synonym for deg, which is currently possible due to lax parsing that ignores punctuation characters after the number.

I've also argued before that pi or pirad should be a valid angular unit in CSS. This would be trivial to add. However, since hues are almost always provided in degrees, this is probably not the best place to make that case.

By the way, the regular expressions used in parseFunction() do also not accept all numbers allowed by CSS Values and Units or CSS Syntax, e.g. scientific notation, and they are not case-insensitive.