pre-commit: Add codespell hook

[alexdewar]

Add codespell pre-commit hook. I've found it useful on a few projects so I think it's probably worth rolling out more widely. It generally doesn't produce too many false positives (it errs on the side of false negatives) and you can always add extra words to .codespell_ignore.txt if it does.

Closes #30.

[AdrianDAlessandro]

Am I interpreting this correctly that this will only apply to this repo and not any repos that are built with this tool? There are two .pre-commit-config.yaml files and this PR only edits the one at the top level and not the one inside the {{ cookiecutter.project_slug }} directory. The same goes for any changes to the workflows, as these appear to be duplicated also.