Texmd

Description
Bug fix for import errors.

Error
When importing the module in Python `3.13` or lower raised exception, caused by `Generator[...]` since the `defaults` attribute of `Generator` is a recently added feature.
Also the package `pybtex` requires `setuptools` but went missing on installation via `pip`.

Fix
Using backward-compatible syntax, `setuptools` were placed in the requirements.

Added options for customizing reference formats.

- Included full LaTeX node structure that allows post-parsing editing of the node tree.
- Some LaTeX nodes which is a sub-class of `TexParentNode` supports node lookup and removal based on rules.
- Restructured the library to have its own `TexParser` and encapsulated parsing state that allows multi-threaded parsing.
- Added support for more complex rules for parsing LaTeX group nodes which allows decorative macros to be recognized in the node type.

Added support to BibTeX.

This version supports:
- Load and parse `.tex` files then write the expressions in Markdown-compatible format.
- Load and parse LaTeX expressions then write the expressions in Markdown-compatible format.
- Customize the Markdown writing backends.