Review of the 8.x-1.x branch (commit 389d4af):
- README.md or README.txt is missing, see the guidelines for in-project documentation.
- The field_formatter_range.module does not implement
hook_help(). See https://www.drupal.org/docs/develop/documenting-your-project/module-docu... .
- No automated test cases were found, did you consider writing PHPUnit tests? This is not a requirement but encouraged for professional software development.