No way to get TokenSerializationType directly

[makotokato]

From https://bugzilla.mozilla.org/show_bug.cgi?id=1503656

Actually there is no way to get TokenSerializationType directly from Token type. If it is TokenSerializationTypeVariants::Nothing, it has nothing() function to get it.

Even parsing CSS that is hardcoded value such as 0px, to get type, we have to create Token object like the following.

let token =
    Token::Dimension {
        has_sign: false, value: 0.0, int_value: Some(0), unit: CowRcStr::from("px")
    };
let token_type = token.serialization_type();

I am happy if cssparser has a function to get TokenSerializationType without creating object like TokenSerializationType::nothing().