LibWeb: Give CSS Token and StyleComponentValueRule matching is() funcs

The end goal here is to make the two classes mostly interchangeable, as
the CSS spec requires that the various parser algorithms can take a
stream of either class, and we want to have that functionality without
needing to duplicate all of the code.

Userland/Libraries/LibWeb/CSS/Parser/Tokenizer.cpp

@@ -207,7 +207,7 @@ Vector<Token> Tokenizer::parse()
         auto token = consume_a_token();
         tokens.append(token);
 
-        if (token.is_eof()) {
+        if (token.is(Token::Type::EndOfFile)) {
             return tokens;
         }
     }