Utility method for detecting that an application using Vaadin Elements is running in development mode

There are no automatic tests as the functionality depends on the host the code is run on (localhost vs others) and the bundling status.

To verify functionality:

1. Run npm install
2. Run bower install
3. Run npm start
4. Open https://localhost:8081/test/index.html
5. Verify that mode is development (green)
6. Verify that importIfDevelopmentMode is run (green)
7. Open https://<yourip>:8081/test/index.html
8. Verify that mode is production (orange)
9. Verify that importIfDevelopmentMode is ignored (orange)
10. Run npm run start:prod
11. Open https://localhost:8081/test/index.html
12. Verify that mode is production (orange)
13. Verify that importIfDevelopmentMode is ignored (orange)
14. Open https://<yourip>:8081/test/index.html
    1. Verify that mode is production (orange)
    2. Verify that importIfDevelopmentMode is ignored (orange)