Add `cspell` checking

[jjspace]

CSpell is a really nice library/collection of tools to add spell checking to code. It's mostly used through the VSCode extension. However it does have a CLI and library version that can be used in things like git pre-commit checks or CI. Longer term this might be helpful to maintain good spelling. Definitely not a top priority but could be nice to add.

Custom dictionary

The key part that's needed to start using the CLI is having a custom dictionary of words that are valid in the project but not in existing dictionaries. #11953 was added to start building that.

When a word is flagged you just have to put the cursor on it and trigger the VSCode suggestion window Ctrl+. for me. Then just make sure you select the cspell.json file to make sure it's added to the repo's dictionary instead of your user or workspace settings.