Make string values for config case-insensitive

Fixes a tiny inconsistency with compiler options where one was all
uppercase and one all lowercase by normalizing to lowercase regardless
of the casing of the user's config.

ghstack-source-id: fe60a3259de89a1b3fdd7475950e16e96cc57f6b
Pull Request resolved: https://github.com/facebook/react-forget/pull/2832