code: private static RubyModule getExtensionsModule(Ruby rubyRuntime) {
          RubyModule asciidoctorModule = rubyRuntime.getModule("Asciidoctor");
          return asciidoctorModule.defineOrGetModuleUnder("Extensions");
      }
docs: Returns the Ruby module Asciidoc::Extensions.
  @param rubyRuntime
  @return The Ruby object for the module Asciidoc::Extensions.