Implement File.extname method

[ggrossetie]

In Asciidoctor.js we are using this implementation:

  def self.extname(path)
    return '' if path.nil_or_empty?
    last_dot_idx = path[1..-1].rindex('.')
    last_dot_idx.nil? ? '' : path[(last_dot_idx + 1)..-1]
  end