.pre-commit-config.yaml: Add codespell

[cclauss]

Fixes #42

1. Commit 1 Add codespell to pre-commit.yaml -- CI tests are not run :-(
   • It unsafe to assume that outside contributors have pre-commit properly installed.
2. Commit 2 Add a GitHub Action to run this repo's pre-commit on all incoming PRs -- ❌
   • Trust but verify
3. Commit 3 Fix typo discovered by codespell -- ✅

[okken]

Very cool. Thank you for this addition.
Also appreciate the demo of adding pre-commit to a github action

[peternewman]

For info, there's also an action already in the marketplace to do all the pre-commit stuff automatically for you:
https://github.com/marketplace/actions/pre-commit

And probably with a bit more error checking.