All state held in that class is apparently now moved to the
ParserCombinator which uses the serialized output

Possibly change all methods in the class to be class/static methods
instead in the future.

Extend DocStringTag to include "all" known types, along with document
how each of them behaves. This also forced a few changes to its
behaviour, which is reflected in the updated tests. The biggest change
is that we now barely test for valid "tag_name" values, since a valid
tag should always have a valid (albeit unknown) tag name.

These will later be used for arrows pointing at errors.

A match literal was already literally a string. Instead, tag matches
with data for clearer information.

Previously the the input json was directly mapped into the output
strings, through different formatters. This changes it so we instead
first build a proper AST of python objects, which is later mapped to
output strings (still through formatters).

This gives us much better type information, and will give way to much
more advanced formatters in the future.

This includes some regressions:
* Docstrings aren't handled as well
* The code lacks any form of hyperlinks.

- Moved most datatypes to own modules
- changed output to take a generator instance for multiple output forms
- Wrote tests for the "parser"
- fixed some bugs revealed in the parser while writing those tests.