No, we aren’t building this as a standalone library. We are utilizing ProseMirror 's library to build the composer specifically for Discourse to support our unique features and formatting.
For now, we are building this editor as a new “mode” of the existing composer. Both modes will support Markdown — the new one just automatically renders it. The current editor will remain available for the time being, and we’ll figure out any long term changes we want to make there based on feedback from users like yourself.