* feat: add basic type stubs
* feat: add types for constants
* feat: add type for `MarkdownConverter` class
* ci: add basic job for checking types
* feat: add new constant
* ci: install types as required
* ci: install types package manually
* test: add strict coverage for types
* fix: allow `strip_document` to be `None`
* feat: expand types for MarkdownConverter
* fix: do not use `Unpack` as it requires Python 3.12
* feat: define `MarkdownConverter#convert_soup`
* feat: improve type for `code_language_callback`
* chore: add end-of-file newline
* refactor: use `Union` for now