Add __version__ to llama_cpp

[janvdp]

I'm trying to add support for llama_cpp but I would like to be able to check the version in Python, in a common way:

import llama_cpp
print(llama_cpp.__version__)

I can send a pull request that:

• adds a version file in the llama_cpp folder;
• lets you run the code above successfully;
• updates the setup.py automatically when building;
• adds a test to check if the version exists.

I have some options for updating the pyproject.toml in a similar fashion. But would love to get your feedback on the whole feature request before trying them.

[abetlen]

@janvdp would be super happy to accept any PR for that!