Convert the project to a TypeScript SDK.
* Add `tsconfig.json` with TypeScript compiler options.
* Add `package.json` with TypeScript dependencies and build scripts.
* Add `src/markitdown/index.ts` to convert `src/markitdown/__main__.py` to TypeScript.
* Add `src/markitdown/markitdown.ts` to convert `src/markitdown/_markitdown.py` to TypeScript.
* Remove Python-specific files: `pyproject.toml`, `Dockerfile`, `src/markitdown/__main__.py`, `src/markitdown/_markitdown.py`, `src/markitdown/__init__.py`, `src/markitdown/__about__.py`, `src/markitdown/py.typed`.
* Update `README.md` to include TypeScript SDK usage instructions.
