SharePoint framework 1.7 is out since November 2018.
Developers gets the opportunity to develop the same app code source for Teams / Classic & Modern SharePoint Page
This tutorial from microsoft is well writtent to cover those steps
I’ve tested it and it is impressively easy to setup.
As per today 11 November 2018, it is only available for dev purpose, not for production purpose. This is really a good feature, together with full app page in SharePoint.
- SPFX 1.7 Released Announce
- Webpart & App in Teams Tutorial
- Full Canvas WebPart Tutorial – Single part app pages