REM length unit is missing in MACROS

[alexrichterxyz]

The CSS "rem" unit is missing here

cssutils/cssutils/profiles.py

Line 91 in 57179fe

'length': r'0|{num}(em|ex|px|in|cm|mm|pt|pc)',

[jaraco]

At first blush, it sounds like an easy fix. Would you be willing to explore it and add support for it and add it to the tests if relevant (if there's already tests for each length unit)?