Convert internals to use mdast for future conversion features (i.e. to HTML), serialization, etc.

[zrosenbauer]

Describe the problem you'd like to have solved

Currently the internals are stored as a "plain object" with no rhyme or reason (just needed to get it working...). This makes it difficult to integrate with prebuilt libraries like those from unifiedjs that would be great for translation from MD <=> HTML and also for serialization.

Describe the ideal solution

Covert the internals to use mdast format for storing the "built" document.

Alternatives and current work-arounds

👎 Build your own convertor, but you'd have to extend the class and expose the internals

Additional context

n/a

[zrosenbauer]

After deep diving, better to output md => mdast vs. building into the lib.

Closing in favor of #12