Add codespell to pre-commit config

[dstansby]

A suggestion to add https://github.com/codespell-project/codespell. I've found it useful in the past.

[paddyroddy]

Just gave this a go. Looks good!

[samcunliffe]

It is great. And we should recommend it for sure.

Not convinced baking it into the pre-commit is a good idea though: I've seen false positives before. And also had it break my code. (Perfectly fine, and trivial to revert... so long as it's not required in order to merge PRs or push commits.)