This can be done through the Drupal UI by navigating to http://localhost:8000/admin/structure/display-modes/view and clicking on the "+ Add view mode" button.
It can be exported by visiting http://localhost:8000/admin/config/development/configuration/single/export. Add it to the islandora/config/install.
@dannylamb is this literally just creating a view mode called "Teaser" and then exporting it? Is any additional configuration required?
@kimpham54 once tested and exported you must add it to islandora/config/install and make a pull request. That should be all
Resolved with https://github.com/Islandora-CLAW/islandora/commit/4b882db0ecbb2fb3f07e1684d53e0494d2695bab.
Thx @kimpham54 and @DiegoPino
@kimpham54 you are the best!++