We’ve created a plugin for ProseMirror that integrates our grammar checking component into ProseMirror. This allows you to add real-time grammar checking into your ProseMirror editor. The code is open source here:
Feel free to do with it as you will even if that involves integrating another grammar checking API.
You can sign up for a license key here: https://prowritingaid.com/en/App/BeyondGrammar
I’d love to hear any feedback or feature requests.