Add message arguments to function-disallowed-list

[glen-84]

What is the problem you're trying to solve?

Setting a custom message for the function-disallowed-list rule.

What solution would you like to see?

"function-disallowed-list": [
    ["rgb", "rgba"],
    {
        message: (functionNameOrRegex) => {
            switch (functionNameOrRegex) {
                case "rgb":
                case "rgba":
                    return "Use HSL colours";
            }
        }
    }
]

[ybiquitous]

@glen-84 Thanks for the proposal. See also #6453 (comment).

I've labeled the issue as ready to implement. Please consider contributing if you have time.