Type annotations improvements (ADHI-4964)
Add init.pyi file describing extension module interfase. Moved code from init.py to _file_read.py to reuse it in init.pyi. Add py.typed.
Unfortunately, LCG comes with mypy <1.0 which makes it difficult to check
typing w.r.t other modules (coca), so mypy.ini
still disables all of them.