LibWeb: Convert CSS Token::m_unit from StringBuilder to FlyString

This value doesn't change once it's assigned to the Token, so it can be
more lightweight than a StringBuilder.

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

@@ -685,7 +685,7 @@ Token Tokenizer::consume_a_numeric_token()
 
         auto unit = consume_a_name();
         VERIFY(!unit.is_empty() && !unit.is_whitespace());
-        token.m_unit.append(unit);
+        token.m_unit = move(unit);
 
         return token;
     }