added testing using jest and the Firebase Emulator Suite.
feature - When executing the backfill function, add a property named duplicate which is set to true if there were duplicate values of the unique field.
feature - Added backfill capabilities so that we can have existing documents in the Firestore collection be checked for field uniqueness.
feature - Update Cloud Functions runtime to Node.js 14.
feature - Add the HASH_FIELD parameter so that we can decide if we want to hash the field name or not, due to contraints on document IDs.
Initial release of the Field Uniqueness extension.