Implement CSS Colors level 4

In #28, a user reported an issue parsing colors with the `/` symbol in them. Here's a minimum reproducer:

```
python -m cssutils.scripts.cssparse -s '.any{color:rgb(0 0 0 / 0.4)}'
ERROR COLOR_VALUE: Missing token for production end FUNC ")": ('CHAR', '/', 1, 22)
ERROR PropertyValue: No match: ('CHAR', ')', 1, 27)
ERROR PropertyValue: Unknown syntax or no value: rgb(0 0 0 / 0.4)
ERROR CSSStyleDeclaration: Syntax Error in Property: color:rgb(0 0 0 / 0.4)
b''
```

Use the minimum reproducer with `python -m cssutils.scripts.cssparse` as the starting check. Trace how `rgb(0 0 0 / 0.4)` is parsed, and consider the work complete when this CSS Colors Level 4 syntax is accepted without the reported errors; the issue names no source file or test.

Written by the indexing model from the issue text.