Need to flag the frequent typo mixing up "an" and "and"

Writing "an" accidentally when the intent is to write "and" is very common. It's one I've spotted in my own issues, PRs, and comments right here.

I'm not sure how common the reverse, writing "and" when it should be "an" is.

Since both words are in the dictionary, the `SpellCheck` linter can't spot them, so a dedicated linter is required.

I've started working on this already.