No support for rem

If I use `rem` as a length unit, I get an "invalid value" error:

`ERROR Property: Invalid value for "CSS Backgrounds and Borders Module Level 3" property: 0.75rem [1:1: border-radius]`

On investigation, it seems that in https://github.com/jaraco/cssutils/blob/main/cssutils/profiles.py, `rem` is not supported as a unit:
`'length': r'0|{num}(em|ex|px|in|cm|mm|pt|pc)',`

Is this something that could be added? I'm not particularly familiar with this module as it's a subdependency so I don't know if it would take more than just adding it to the pattern.

Start in cssutils/profiles.py at the length pattern shown in the issue and inspect how CSS property values are validated. Check the existing handling of length units and verify that a value such as 0.75rem no longer produces the reported invalid-value error.

Written by the indexing model from the issue text.