Add auto spell check as part of pre-commit checks

[drbenvincent]

We had a report of a spelling mistake "allot" (should be "a lot") in this notebook https://causalpy.readthedocs.io/en/latest/notebooks/iv_pymc.html

Let's add a spell check as part of the pre-commit checks.

There might be other good options, but this looks reasonable: https://calmcode.io/course/pre-commit/spelling (GitHub link: https://github.com/codespell-project/codespell)