chore(package.json): add Jest as a dev dependency
chore(package.json): add Jest as a dev dependency

- Add Jest as a dev dependency
- Update app description text

Based on:
https://github.com/ionic-team/stencil-app-starter/commit/6ed1d2fa7e105b518fa916c4885da28c02111c8c

d2s/learning-stencil,d2s/learning-stencil,d2s/learning-stencil