Implement CSS Colors level 4

[jaraco]

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''

[jaraco]

Apparently this syntax is part of CSS Colors Level 4. It would be great if someone would look into implementing this syntax and any other syntax required for colors level 4.