Proposal: Change various visitor implementations from structs to classes

[charrondev]

This project implements various visitor classes that have methods for generating or rendering each type of markup.

I'd like to switch these structs to be classes and add the ability to replace their implementation in the ParserBuilder.

By making them classes they can be extended while preserving most functionality.

For example I have a fair of experimentation I'd like to do with tables in the near future that likely won't be immediately upstreamable and would prefer not to create a full duplicate implementation of all of the other visitor functions.

[zhgchgli0718]

Any adjustments that make the structure more streamlined and stable are highly welcome.
I can assist with reviewing and adjusting test cases.

Additionally, if you are interested, you can also join as a maintainer for this project.
Thank you.