The base converter has a default method handles? that always return true:

https://github.com/asciidoctor/asciidoctor/blob/c55fc9fea20d57f14e9552073dab5a27a4728f71/lib/asciidoctor/converter.rb#L82-L84

When registering a Converter from plain JS we should bridge this method.
We should also assign handles? function if the converter has a handles? or handles function define.

In other words, handles? should always be defined either with the default implementation that always return true or with the actual implementation provided by the converter.