Not too well-versed with composer but it might be worth calling out that you need to specify "minimum-stability": "dev" within the root of composer.json (of your main project) to be able to pull in your extension. Was getting this issue for a while while trying to use composer require.

Issue: Root composer.json requires shaybox/grammarly *, found shaybox/grammarly[dev-master] but it does not match your minimum-stability.