Add a `serve` command to start a CORS-enabled Flask server for file conversion.
* **New Flask Server**: Add `src/markitdown/server.py` to define a Flask server with CORS enabled and a route to convert files to markdown.
* **Dependencies**: Update `pyproject.toml` to include `flask` and `flask-cors` as dependencies and add the `serve` command to the `[project.scripts]` section.
* **Documentation**: Update `README.md` with instructions on how to use the `serve` command.
* **Tests**: Add tests in `tests/test_markitdown.py` to verify the functionality of the `serve` command, including handling both URL and file POST data.
In case we decide not to have `serve` command we have [markdown-converter](https://pypi.org/project/markdown-converter/).
