Rule to ensure `process.env.NODE_ENV` is only equality-tested against valid values

I want to make sure that no one accidentally uses a condition in their code like process.env.NODE_ENV !== 'productn'.

The rule would error if process.env.NODE_ENV is equality-tested, or inequality-tested, against a value that is not a literal string of one of the allowed values. Allowed values could default to the conventional 'development' and 'production', and could be configured by the user.

Thoughts? Should I go ahead with a PR?