Make Asciidoctor::Extensions.register a module_function

[vivien]

Because register is defined inside class << self, it is currently not possible to write an extension like this:

require 'asciidoctor/extensions'

include Asciidoctor::Extensions

register do
  inline_macro :git do
    process do |parent, target, attrs|
      command = "git-#{ target }"
      create_anchor parent, command, :type => :link, :target => "https://git-scm.com/docs/#{ command }"
    end
  end
end

Much of the stdlib uses module_function, so we get used to it working like that. I think this seems more intuitive for anyone, instead of calling explicitly Asciidoctor::Extensions.register.

Can register and friends be considered module_function instead?

[mojavelinux]

The simplest way, of course, is to replace class << self with module_function. But I'm not convinced we want to export all the functions.

I came up with away to selectively export, while still keeping the methods defined in self. See #3198 and let me know what you think about that approach.

I wish Ruby had a way to promote a static method to a module function, but it doesn't seem to work that way. My workaround seems to be the next best thing.

[mojavelinux]

I have to ask, is this really saving you that may characters? The Extensions module doesn't really provide utility functions, so promoting them to top level functions feels out of place. I could see it if these were pure functions, but they aren't.