Add spell checker

Hi there,

by integrating [Vale](https://github.com/errata-ai/vale), this project already features a prose linter. On top of that, we might also want to think about integrating a spell checker. Hereby, I am proposing [PySpelling](https://facelessuser.github.io/pyspelling/), together with [unanimous](https://github.com/resplendent-dev/unanimous).

With kind regards,
Andreas.
