Add Python type stub file

[N-Wouda]

This PR closes #1866. It adds a Python type stub file that covers the content of the _core native extensions.

I generated these stubs using mypy's stubgen, as follows:

stubgen --parse-only -o . -m _core

I then checked the stub file for any errors, and although it can always be improved, I think this'll suffice as an initial default. It's certainly enough to provide Python users with meaningful type information and autocompletion when working with highspy.

[N-Wouda]

One thing I'm not entirely sure of is how to keep the type stubs in sync with the actual extension going forward. In my own projects I write these files largely myself and thus also keep them up to date, but I can imagine you might not want to spend a lot of (human) time with this. In that case doing stub generation during Python package builds could be worth investigating?

[galabovaa]

Thank you! We will keep in mind to update this when we update highspy, and, as you say, it may be worth automating. Looks good to me!