fix O(n^2) behavior when checking for duplicate attributes (libgumbo)

[flavorjones]

Please describe the bug

Originally rubys/nokogumbo#144

Code was added to limit the number of attributes supported per element to prevent DoS attacks: rubys/nokogumbo#143

That safety limit is here: https://github.com/sparklemotion/nokogiri/blob/main/gumbo-parser/src/tokenizer.c#L792

It would be great to support more attributes by addressing performance concerns in the implementation.